Does anyone manufacture aftermarket floorplates for the Ruger M77 series to allow a bit more capacity? It would be nice to be able to stuff an extra round or two in the box...

i don't know about the extra capacity, but there were some steel floorplates and triggerguards available a little while ago. Check with Brownells. - Dan

I dug through my files and found something that might help you. Several years ago in Shotgun News there was a company that was called:
Carolina Arms Manufacturing
P.O. Box 3292
Salisbury, NC 28145
1-704-637-0203
1-704-636-7713 (FAX)
They advertised "Ruger M-77 drop magazine conversion. New floorplate, button catch, trigger guard and semi finished stocks in four styles available for Mauser, Ruger M77, Remington, Winchester and Howa. Steel grip caps in three styles and two sizes."
I don't know if they are still in business, but I do remember they offered these for the short and long actions only, no RSM length. You might try and contact them and if you are successful post a reply here so that others might learn about them.
Good luck!