At the present time, the only clue I have for Mrs. Howard comes from her son William's obituary (see below).
1st March 1843: Death Notices of Ontario...
On the 23rd of January, William Howard, of the township of Grimsby, in the 67th year of his age. He was brought from England to America at the time of the Revolution, by his father, who was a Surgeon, in the Royal Navy and who was slain in the commencement of the war. His mother was afterwards married to a Lieutenant McDonald, with whom he removed to Upper Canada when but a boy. He leaves a widow and seven children.
Assumption #1: Mrs. Howard was born in England
Assumption #2: She and Mr. Howard lived in America
Assumption #3: She married Lieutenant McDonald (McDonnell, etc) in America
Assumption #4: She moved to Upper Canada ca 1785 (William was born ca 1776)
Do I need to start my search in Canada/Upper Canada? Perhaps a better place to start would be a couple of British FHL films:
Information from the Olive Tree Genealogy site indicated that there are microfilmed records entitled "Applications for Pensions for Widows and Children of Loyalist Officers, 1755-1908," at the Family History Library. A search of Film # 857997 (mentioned at Olive Tree) at FamilySearch.org produced this title:
Certificates of birth, baptisms, marriage and death, : with wills, administrations, statements of services and personal papers of officers and their families...alphabetically arranged, 1776-1881, plus eight bundles of similar certificates, 1755-1908. (W.O. 42/1-51)
A search for both the Howard surname as well as McDonald might be in order.
Bundles 429-450 Horneck-Howes FHL BRITISH Film 860091
Bundles 19-126 Macauley-McDonnell FHL BRITISH Film 857902
Bundles 126-146 McDonnell-Macdougall FHL BRITISH Film 857903
A sub-set included Loyalist Regiments:
Bundles 1-8 McAllister-Robinson FHL BRITISH Film 866502
I don't know if the widow of a surgeon in the Royal Navy would be included, but perhaps she would be as the wife of Lt. McDonald (and would be recognizable if her former name or her son's name was included in the file).