In the onset of the game, technology was thoroughly proliferated. So much so, that there were no "factional" mechs or tech. Certain factions were known to have affinities for certain things, sure, but a Capellan Vindicator or a Kurita Dragon in Davion hands is going to be maintained just as easily as any other mech (as far as "easy" goes in the tail end of the Succession Wars where parts often need to be found via a "scrounge" skill check..)
Not only are mech chassis all thoroughly proliferated, so are the components. There's no functional difference between a Diverse Optics or a Lushann laser, nor a Shigunga vs a Sureshot missile launcher. Whatever differences there are between manufacturers' components doesn't manifest at the boardgame level of granularity (a medium laser is a medium laser, a SRM 6 is an SRM 6, etc) and even at the RPG level any difficulties in mixing and matching components at the "fluff" level are either easy, or only at the whim of the GM depending on the edition. And honestly it can make sense that the entire industry of the Inner Sphere build to the same compatible standards. Even before the necessity to use whatever's on hand during the Succession Wars, before then the monolithic Star League dominated all it touched. From a military industry perspective, who back in that day was going to invest in R&D for tech that isn't compatible with SLDF standards? Once everything military ended up on the same page, it's easy to stay that way.
With centuries (perhaps many centuries, if the Hegemony influenced a Sphere-wide standardization even before the birth of the Star League) of inertia, it makes perfect sense to me that the nominally proprietary "factional" technologies of the mid 31st Century are also all indirectly compatible, since they'd obviously be built to be compatible with each faction's existing base, which are all in turn-cross compatible. In this period where new tech are coming out to this faction or that one, it might be challenging for a merc tech to integrate or repair a newly fielded invention, sure. Technical documents might be held closely to the chests of the respective factions, but I kind of don't think so. It just doesn't fit with the paradigm that's been stretching back to the Age of War. What purpose would it serve, for example, for the DCMS to attempt to classify Streak-SRM4s and -6s? Once everyone else *knows* it's possible by seeing Kurita units fielding them, they can simply engineer them themselves. Or steal the specs. It's more profitable to sell the specs. E.G. what the Sharks/Sea Foxes do with Clan tech :D We also see in lore that manufacturers also routinely offer contracted support. Just off the top of my head, reference the fluff for Manticore in TRO 3026 and Conditterie in TRO:3145mercs. If a merc outfit in the mid 31st century purchases some brand spanking new factionally-restricted tech, odds are they can also purchase the support to maintain it.
Of course, post-Jihad everyone's factional toys became thoroughly proliferated again to everyone (up to and beginning to include Clan-tech) that by the Dark Ages we're again in basically the same starting point as the Succession Wars.