I posted the following to rec.music.folk in April 95 when there was little non-institutional access to internet and I had to "gate" through from a FIDOnet, Electronic Bulletin Board. Just 4 years ago and FIDOnet essentially gone as well as all its folklore & lexicon. "sysop" "BBS" "sig" (not Signiture) "tagline." Seems ages ago.

This is 100 years later than Bruce's reference, but shaping up to the mainline song:

In Thomas D'Urfey; _Pills to Purge Melancholy_ of 1719 or 1720, page 56:

TOM a BEDLAM

Forth from my sad and darksome Cell,
From the deep abyss of Hell,
Mad tom is come to view the world again,
To see if he can ease his distemper'd brain.

Plus 11 verses, many of which include odd classical references such as

Last night I heard the Dog-Star bark
or
Lymping Vulcan had the gout

ending with

The man in the Moon drinks Clarret,
Eats powder'd Beef, Turnep and Carret:
But a Cup of old _Maligo_ Sack
Will fire the Bush at his back.

I think this is pretty clearly the main root of the Steeleye version.