Realistically your best defensive weapon is speed. Flee your foe using your multi-modal powers of confoundment. Almost anything that you use to fight while riding will involve a sudden and startling de-biking (which reminds me, we need boffer lances for jousting!). U-lock justice works best at low speed; faster and you'll crack a wrist.
Paintball guns full of paint stripper belong only in the empowerment-fantasy that spawned them. You'll miss, dissolve the skin from a little old lady, and get shot by the armed police. They're in the same category as the combat tailwhip.
If de-biked, use the bike as a barrier to enable withdrawal. Last ditch, throw it into them, bikes are tangly pointy things. Bikes make interesting impromptu Lochaber axes.
The LAPD
Tactical Firearms Guide for Bicycle Officers (I'm not making that up) recommends dismounting with the bike between you and the bad guy, drawing your sidearm and bracing against the bike, using it as a bipod.
One I get my penny-farthing fu up to the level of the Secret Chiefs, I shall be taking advantage of the height with a cavalry sabre.
