A few #reaperdaw region tail length scripts offer a thing I want w/ a thing I don't so... I designed one in LUA that creates a single region with a tail length and name for selected items, both entered by the user. It checks that you enter a non-negative "number" for the tail. 
I use this to avoid extending region lengths, after creating them, on content w/ reverb that extends beyond the selected items. I'll still listen to adjust, and trim in SoundForge. Hopefully this simple script is helpful. If this exists already, sorry! Reapack = convenient! 
