Are there any online wallet providers which allow entry of a pin or password using an on-screen keyboard (simlar to what Liberty Reserve's site provides?)
  http://anti-keylogger2.software.informer.com/wiki/#On-screen_keyboards

I was wanting to make a payment using my MyBitcoin account while away from home, and didn't want to enter my password on the only computer I had available to me as I didn't trust it.

I would trust an on-screen keyboard on even a public kiosk style computer (for a smaller balance account) but  I wouldn't dare trust it entering my credentials via the keyboard. [Edit: See nanotube's comments.]