I'm not familiar enough with the DM chat NIPs to answer this with any confidence. @Vitor Pamplona and @hodlbod and @fiatjaf all know more than me about multi-party DM.
I don't know if I follow? Can't you just send each as a separate event?
You could but if a big stack of DMs goes to a large relay they can rate limit or block, as it can look like spam. cc: npub1ymt2j3n8tesrlr0yhaheem6yyqmmwrr7actslurw6annls6vnrcslapxnz