This is just a wild guess off the top of my headsince I am not able to play around, but wouldn't placing the comment form inside the <MTEntryIfAllowComments></MTEntryIfAllowComments> tag do the trick?

Hmmm, my guess is that Shawn's solution won't work, unless you could change the source for a single entry. (perhaps as an extra category?)

mind you, I don't use MT... :)

Promo, that code IS the code to use. If you allow comments, the comment box will appear. If you do not allow comments, if the comment box is in the tag, the box will not appear. It's how my comments are for my site.