The DCMS (and perhaps other Successor States) could have lots of Mech chasses just as the US has a boneyard full of older aircraft. What they probably lack is people and funding to pilot and properly support them with tanks, infantry, techs,
logistics, etc.
I agree that idea is reasonable for the Star League, and the Pax Republica, but I have to agree with SCC that it's not congruent with the scavenger / meat-grinder periods in between.
By the way, the DCMS Night Stalkers, AFFS Chisholm's Raiders and FWL Romanov's Crusaders are better examples of units formed with privately-owned Mechs.
Y'know, given that the Night Stalkers were funded by rich members of the business world, and that Chisholm's Raiders are described as "rickety," and that both are light units which were formed from used 'Mechs, I think they're a fair sign of just how scarce 'Mechs are in the secondary market.
I don't recall anywhere in the fiction or in the fluff that a "you keep what you kill" policy is in effect on the individual scale. In fact more often than not we see that the units bargain hard for salvage rights.
It shows up in TR:3025. I'd expect it to show up in the Notable Warrior entries of later TROs too (though I don't know if it actually does). I've recently reread parts of the Warrior Trilogy, and I think I remember it also being mentioned offhand in dialogue there.
In any case, individual prize rights apply to how loot gets distributed
within the unit. It's a separate issue from, and would occur after, accounting for whatever loot the unit as a whole is contractually obligated to pass on to other units.
If grandma is in a position to make such a claim she should clearly be committed.
Having multiple ships in sight could help intimidate an opposing ship into surrendering, or influence its maneuvering in a way which makes it easier to capture. If some MechWarrior tries to claim that
they kept the enemy in the fight so long that enemy had to find somewhere safe to pee - which is to say, they contributed neither materially nor psychologically to the actual capture of the 'Mech - they should be committed right along with the grandma.
Edit: possibly you are misunderstanding the scenario. The MechWarrior isn't cracking his cockpit open
in the middle of combat, he's gone off somewhere quiet away from the battle zone. "Safely away from the battle zone" *isn't* a crazy place to find grandmas. BattleMechs are fast and mobile; villages containing grandmas are not.