>Has anyone done this and care to share their ideas/experiences/code? Can printers stack envelopes like regular paper?

Disclaimer: I've not done this, but have considered it in situations where it would be beneficial to the users.

In the EXPR field of the first record of the FRX there are stored a veriety of printer-related settings; here's a sample:
DRIVER=winspool
DEVICE=HP LaserJet 6P
OUTPUT=LPT1:
ORIENTATION=0
PAPERSIZE=1
ASCII=1
COPIES=1
DEFAULTSOURCE=15
PRINTQUALITY=600
COLOR=1
YRESOLUTION=600
TTOPTION=3
COLLATE=1

The one you are interested in is PAPERSIZE. Sizes 1 to 255 are predefined, but I don't know where to find a list.

Do a UT search for PAPERSIZE; it returns 459 messages. One of them may lead you towards a solution. Here's one: Re: Deleting printer information Thread #1031994 Message #1031999 - and another: Re: Reference of Values for PaperSize Thread #1023585 Message #1023588 - and look at this thread too: Setting papersize to 256 causes problems Thread #324869 Message #324869

HTH,