At this point I am laying money on identifying Elizabeth (second wife of Col. George Beall (a/k/a George Beall 2nd)) with Elizabeth Waring Beall, who was the widow of Joshua Beall (Beall-807).
Excursis: Some have maintained that she was the daughter of Basil Waring III who died in 1776 in PG County, as asserted in the section of "Colonial Families (etc.)" on the Warings, but this seems most unlikely, because Elizabeth the daughter of Basil Waring III was unmarried as of the 1776 date of his will. Thus she would have been likely born no earlier than 1756. If so, she could not be the widow Elizabeth (Belt) Waring Beall (see discussion below) who was over 45 years old in 1800. So I vote with those who maintain that the "Elizabeth Waring" who married Col. Joshua Beall as his second wife was the widow of Basil Waring III; hence Elizabeth (Belt) Waring, rather than a spinster Elizabeth Waring.
Back to the main issue:
On the 1790 census of Prince Georges County, there are two men named Basil Beall. One, who lived near Lloyd Ball and Andrew Beall of Ninian, is shown as having one slave. Clearly this is not the testator of 1795 who left a widow Elizabeth Beall, as in his will THAT Basil Beall named at least ten slaves. So the 1795 decedent must have been the Basil Beall who was enumerated near the far end of the County, with 15 slaves.
Notably, the Basil Beall who wrote his will in 1795 had no posterity, as no children or grandchildren are mentioned in his will. Rather, he left his entire estate to his wife, and upon her death to her sister Ann Tarvin, subject to the eventual emancipation of his slaves and regular bequests to the slaves in the interim; such emancipation to be managed by his friend Evan Thomas to the extent not accomplished during the lifetimes of the widow Elizabeth and her sister. Most importantly, Basil identifies himself in the will as Basil Beall "of Charles County, Maryland." Consequently, we are talking here about a family who must have lived about twenty-five to thirty miles from Georgetown; and therefore not likely to have been in the same social circle as Col. George Beall.
In contrast, on the 1790 census of P. G. County, Joshua Beall was enumerated next to "George Beall 3rd." Since this census is organized alphabetically, there could actually be a few homes between those of Joshua Beall and George Beall 3rd. (FS image page 4, second sheet). Of course, I have to concede that toward the end of the page is a Basil Beal (the one with 15 slaves), but there are MANY families starting with B in between, so this would seem to be the Basil Beall who probably lived some 25 to 30 miles from Col Joshua Beall and George Beall the 3rd.
Joshua Beall's home, I gather, would have been in or near what is now the Mt. Pleasant neighborhood of Washington, DC, as he inherited his father's lands between Piney Branch and Beaver Dam Branch (which enters the Anacostia River just south of the Rte 50 bridge). The north side of Mt. Pleasant drains into Piney Branch. (I am placing Joshua's likely home toward the west end of his lands in part because it's a more salubrious neighborhood than the Anacostia swamps, and one would naturally want to locate convenient to the resources and sociality of Georgetown.)
I find it significant that on the 1800 census of DC (that part formerly in PG County), which is not alphabetized, George Beall 3rd appears just 7 doors from Elizabeth Beall (single woman over 45 year old), and none of the intervening heads of household has a surname starting with "B." In other words, the widow Elizabeth Beall on page 156 (crayon numbering) of the 1800 DC census must be basically in the same spot as Joshua Beall's farm appeared on the 1790 census.
Now Col. George Beall was enumerated in Montgomery County in 1790, and his son -- also called "George Beall 3" was found some six doors away. This was obviously the portion of Montgomery County that was taken into DC. So, in 1800 Col. George Beall is found in Georgetown, DC, with no woman in the home. From the Beall home in Georgetown it would have been a short horse ride to cross Rock Creek to pay his respects to his cousin Joshua's widow, Elizabeth (Belt) Waring Beall. As her late husband had been a Colonel, and the owner of thousands of acres of the territory of the new national capital, the widow Beall would have been of nearly equal social standing with Col. George Beall of Georgetown. In short, she was a perfect match with her "new" colonel.
So that, imho, is the "Elizabeth Beall" whom he married later that year.