Every now and then there's a discussion about bringing in replacement characters in OSR games. There was one recently on the reddits that brought this back to mind. The consensus is that you should find excuses to get new PCs into the game. But the poster points out that this distorts the resource game, and then people amend their position so that obviously you should have replacement PCs come in with partly-depleted resources.
As for me... I'm thinking this sounds like an awful lot of DM fiat. I eagerly await the "rulings, not rules!" in the comments. But seriously, there are a lot of things that I don't like about the... 3rd? 4th? wave OSR but one thing I do like is the focus on little rules subsystems, "procedures". See also Arbiter of Worlds' discussion on rulings establishing precedent and evolving into rules.
Anyway. What would a system for adjudicating the arrival of new PCs look like?
Before designing one, it's worth checking whether we already have such a system in place but have failed to recognize it. And I think Wandering Monster tables that are heavy on demihumans and "rival" adventuring parties could easily serve this purpose. I have never had a good explanation of the point of having 30% of B/X's dungeon level one encounter table be demihumans and humans. But maybe these encounters are intended to be a source of replacement PCs. Then the deeper you go, the less frequent these encounters become and the harder it becomes to replace your losses in the dungeon. Using the first level's friendly table to gather reinforcements pairs interestingly with using it as a safe haven to rest in for expeditions down to the second level. The sharp drop off in potentially-friendly results on the encounter table as you level is interesting - maybe by the time you're going into the third dungeon level, you're expected to have hirelings rather than "living off the land" for replacement characters. And then again, in the wilderness you have the Men table, but usefully-leveled results vary in frequency by terrain type.
So what's the procedure here? Roll a random encounter with demihumans or adventurers, get a reaction roll better than Hostile, and then you can smuggle your replacement PC into that game that way? And maybe they come in with fairly complete resources, but you had to take a significant risk (an encounter roll) to get them. And if you're a party with multiple members down, maybe you make a bunch of noise to provoke encounter rolls in the hope of friendlies rather than the conventional wisdom of quietly trying to escape. It's a high-risk double-or-nothing play but... having that choice, between the quiet approach and the loud approach seems like it could make for some interesting gameplay.
The wildly-diegetic angle here would be to roll NPC parties completely straight, and then if they're not hostile on the reaction roll, allow players to pick an NPC to start playing. Might be exploitable if you go on "recruiting" expeditions in high-level areas though.
I guess I'm not convinced that some delay on the arrival of replacement PCs is an ultimate evil, particularly if you let players who are out of characters continue to contribute to the party's problem-solving discussions (voicing someone else's henchmen, perhaps). Particularly if a caller is being used, where people aren't acting out on their own behalf.