I've moved the research plan / search results from the question into a self-answer.
Somerset Marriages
A search of FreeBMD turned up a candidate for a marriage registered Mar 1845 in Wellington Reg District for a Stephen Rossiter. A Postem on that entry says:
Stephen married Ann COURT (bap 21 Apr 1822, Chipstable, Somerset). Her
details were added later to the marriage index so don't seem to come
up with normal search.
A marriage between two people in this time and place with those names would be consistent with the elder daughter named COURT and the younger daughters named ROSSITER in the 1851 Census.
Somerset Baptism Registers
ROSSITER Stephen 11 Jun 1826 Roger Sarah (Reg No) 1159
taylor
ROSSITER Harriett 22 Mar 1846 Stephen Ann (Reg no) 1257 labourer
ROSSITER Fanny 30 Apr 1848 Stephen Ann (Reg no) 1410 labourer
Sister Sarah is born too late for the coverage period in this collection.
- Searching again on FamilySearch, I found results in England Births and Christenings, 1538-1975:
https://familysearch.org/ark:/61903/1:1:NXFD-C68
Stephen Rossiter son of Roger and Sarah baptised 11 Jun 1826 in
Wiveliscombe, Somerset, England
On GENUKI the 'Transcriptions from "All Saints" Parish Registers, Chipstable, from the E Dwelly transcriptions' as transcribed by David Cheek lists under baptisms:
COURT Anne 21 Apr 1822 James Jane (Reg no) 103 labourer - (no comments)
Burial Registers
PolyGeo and ColeValleyGirl suggested other useful links to Somerset Parish Registers. I will search for burial records to fill out the timeline for this family.
Other Somerset Census Records
(Updated 2016) Further review of this family suggests that Stephen and Ann (Court) Rossiter moved to Stokenham, Devon around 1855 (in between the births of sons James b. 1854 and Samuel b. 1856). So the only Somerset Census records needed are for Stephen Rossiter and Ann Court in 1841.
I searched the category "Census, Land, and Substitutes" at Find My Past for "Stephen Rossiter" in Wiveliscombe, and the only result was an 1841 Census record:
Langley, Wiveliscombe, Wellington, Somerset, England
- Roger Rossiter Male 70 Somerset, England
- Sarah Rossiter Female 60 Somerset, England
- Henry Rossiter Male 22 Somerset, England
- Charles Rossiter Male 20 Somerset, England
- Stephen Rossiter Male 15 Somerset, England
Unlike later censuses, there is no house name or address I could search for.
Electoral Rolls
Searching for Roger Rossiter in Wiveliscombe brought me back to the same 1841 Census record, but yielded no hits from the Electoral Rolls. Similarly, searching for Henry returned only this 1841 Census record. Searching for Charles gave this census record, and one from 1851 which had Charles as Head, plus his new family, and widowed mother Sarah.
There were no hits for any of these males in this ROSSITER family in the Electoral Registers, so it seems this line of inquiry is a dead end.
Update: December 2016
Since I wrote the original question, Ancestry has added a number of collections which have allowed me to fill out some of the timeline for this Somerset family.
Of particular interest are two entries in Ancestry's database UK, Poll Books and Electoral Registers, 1538-1893 for a Roger Rossiter in the Hundred of Kingsbury, West:
1832 image for Wiveliscombe parish and record page
Name: Roger Rossiter
Poll Year: 1832
Hundred: Kingsbury, West
Parish or Rectory: Wiveliscombe
County: Somerset
The entry reads:
Rossiter, Roger, Wiveliscombe, lease for years of house and lands, part Langley Marsh
and
1834 entry for Wiveliscombe parish and record page
Name: Roger Rossiter
Poll Year: 1834
Hundred: Kingsbury, West
Parish or Rectory: Wiveliscombe
County: Somerset
The entry reads:
Rossiter, Roger Wiveliscombe house, fields, orchards and gardens at Langley, (tenant) Roger Rossiter
This general location is consistent with the family headed by Roger Rossiter who was enumerated in Wiveliscombe in the 1841 Census (Piece 950 Book 10 Folio 31 Page number 54) which was discussed earlier.
I also have a search result for Roger Rossiter (as Occupier) from the Tithe and Landowner records at The Genealogist (dated 1840).
Further Research
Ancestry has also added more Somerset Parish register images (see below), and I have been able to download many of the baptisms for this family.
With the information from the baptism registers, census records, and other records gathered since I wrote the question, I can make a research log and a fresh plan to look for specific individuals in the electoral rolls at Find My Past. Searching for both "Wiveliscombe" to find the parish and "Rossiter" gives me 28 results (most in West Somerset), which is a reasonable number to look at page by page.
I am grateful for the suggestions in the comments for other links to Somerset parish registers to correlate with the records for this family. I'm including the links in the answer so they won't get lost.
The newly-updated (Dec 2016) collections on Ancestry that will be consulted in further research for this family are from Somerset Archives & Local Studies (South West Heritage Trust):
- Somerset, England, Church of England Baptisms, Marriages, and Burials, 1531-1812
- Somerset, England, Church of England Confirmations, 1843-1913
- Somerset, England, Marriage Registers, Bonds and Allegations, 1754-1914
- Somerset, England, Church of England Burials, 1813-1914
- Somerset, England, Church of England Baptisms, 1813-1914