Yes you can use the expedited service for your MRP passport even though your passport was issued from Gorkha.

Send the complete filled out application form to gorkha (Jilla prasasan karyalaya), they'll verify and stamp it in the form and come back to KTM and submit your application.

You're done !

Those who said you can't do it probably heard from their friends friend !

How do i know this ?
I did it couple months ago !