I, personally, have split a simplex RS232 signal to multiple sources simply by splicing. I don't know if it's up to RS-232 spec (obviously not, since RS232 is a duplex protocol, but you know what I mean), but it worked on mutiple disparate hardware recievers for the signal. In my particular app, it really was simplex, as it was receiving a feed from a satellite dish with no way to send data back, and it was connected to a black box provided by the feed source, and split off to also feed a PeeCee running QNX, a Sun Sparc machine running Solaris, and, it seems, something else, too, but I can't remember what now.

Regardless, it's possible and that's probably the way you should try first.