Ask and ye shall receive!
(...honestly, I was a little surprised I managed to do it

)
http://te4.org/games/addons/tome/crossbowsforkThought I would take a look into it. Turned out to be easier than I expected. Longest part was getting the crossbows and bolts to drop so I could test them
This is a fork of the original by Sebsebeleb. Much of the code is his, so credit goes where it is due.
I took out the parts that were conflicting with the new version of ToME.
Crossbows and Bolts will drop in game and appear in shops. They will pick up egos from Bows and Ammo.
Crossbows and Bolts require strength.
I made some adjustments.
The old version, at least as far as I could tell, limited the ammo on all Bolt quivers to 1 shot. I upped that considerably and made it a random range, but it is still lower than arrows.
Bolts do less damage than arrows, but have a slightly higher crit rate and much higher APR.
Crossbows still fire slower than Longbows.
Crossbows will now gain Mastery from Bow Mastery, have an accuracy bonus (APR, same as knives), and should work with all bow skills.
Though, I think they are more designed for melee Strength characters to have in the second equip set in case they get pinned or can't close distance.
Archers and TWs are probably still better off with bows, but honestly I have no idea how they stack up late game.
If anyone is interested in them and notices they are way off balance, let me know and I'll try to tweak things. The APR seemed really high, but again I'm not sure how that stacks up late game.
There are no artifact crossbows or bolts, but if there is more interest in this I could probably look into whipping a few up. Also they still use the same graphics as Longbows and Arrows because I have zero (point zero zero) artistic skill.