Based on http://ubuntuforums.org/showthread.php?t=1037201 I was able to figure this out. It's certainly not obvious!
I'm also using XSane (version 0.998 on 64 bit Ubuntu 12.04 LTS).
I have two printers, one is USB ink jet; the other is network laser. Both use the TurboPrint drivers, but I don't think that matters.
I found the clue is to visit the CUPS page in your browser (http://localhost:631/ by default). Click the "Administration" tab then "Mange printers" button.
You'll see the left column called "Queue Name". Copy that que name for one of your printers.
In XSane, Preferences|Setup|Copy you'll see it probably has "new printer"in the "Name" field. Paste in your printer queue name to replace "new printer" (although I suspect you can put any meaningful name there).
In the "Command" field, enter:
In my case, I have:
Click "Apply". That should be it. Repeat for other printers and their queue names. You might need to click "Add printer" to get a new default template.
lp -d iP4850-A4-TurboPrint
Note that if you've been clicking "Add printer" often, to add a printer, expecting something meaningful to happen, you'll probaly find you have simply added numerous printers called "new printer". You can check that in the top drop down printer selector. If there are excess printers there, pick one, then click "Delete printer". Repeat for other excess printers.
But I'm new to this, so there may be a better way.