This might be a slightly strange problem here, but,

I have a Canon printer (860i) which uses 2 black ink tanks (3e and 6), and the text black doesn't print (this isn't the problem, I do sort of have a way round this)

The way round this that we currently use (on windows) is to set the default paper mode to photo paper - that way, everything that is printed is automatically treated as being photo quality, and uses the working black.

However, I have not been able to recreate this fix under Ubuuntu (8.10). Every option I have tried tries to print blank with the non-working black. Does anyone here have any suggestions as to how to trick the printer into only using the photo black tank?

What driver are you using? A Google search for this printer turned up a special Linux driver that has been developed for these Canon printers.

I'm using CUPS+Gutenprint v5.2.0-rc1

I have tried hunting for and installing other drivers, but never managed to get anything to work.