Hi. I wrote the iP90 review, but I don't have Docs To Go on my Treo 650, however if a program does not have a native (built-in) "send" function like the ones you listed, then you probably need to get PrintBoy. If it has export to memo pad and the export is formatted properly then you could print it from there directly over Bluetooth.
Printboy::
http://www.mobihand.com/treocentral/product.asp?id=365
Take a look at it. I believe this will get you where you want to go. You could always try it with another Bluetooth enabled printer, if you have access to one, or use some demo unit at a store somewhere, if they have one turned on that you can demo. If that does the job, then it should do the job with the iP90.
You ask a good question.

I don't have a definitive answer for you.

If Docs To Go doesn't have a "send (over Bluetooth)" feature like those ROM apps do, then I think Printboy is your solution.
If I can find the time, I'll get it myself and see if it'll let me do things like print out Web pages from Blazer, or anything else that doesn't have a "send" function built-in. I've never used Printboy but i assume it has those capabilities. It's hilarious how many Palm programs have no print menu item or button. I'm looking at Word on the 700w and there's nothing in it to tell it to print. Very strange.
Harv