If it's a pick up game, or set in a mixed-tech era (i.e., Dark Age), or if it's a high-level game and the players are all making manufacturing decisions for their factions, then I wouldn't question it.
It would be very, very strange to see such a model mass-produced during the canonical FCCW or Jihad however. A little bit because they dislike stooping to "barbarian" tech, and a little bit because I don't think they see ProtoMechs as one-shot platforms, but mostly I think regular Clan LRMs are good enough they wouldn't bother.
Maybe make RLs available around the same time that the ProtoMech Autocannon spreads beyond the Blood Spirits.