IPX OS that supports serial DCD/CTS/RTS for a GPS/PPS NTP clock

Wow there's a lot of three letter acronyms in that subject.

The recent thread here about a couple IPXes in search of a good home reminded me about a question that's been bothering me.

I'm trying to make an NTP server with a GPS input that puts a PPS signal on one of the control lines (CDCD, CTS, or RTS).  The IPXes were previously running Debian but they dropped support for the IPX architecture completely.  I tried OpenBSD which still has support but somehow they've managed to mangle serial support and now only TX/RX is supported but none of the control signals are available.

Does anyone have a suggestion for an OS that supports full serial, control signals and all?  I heard mention of NetBSD once but I don't remember the details.  My one copy of Solaris doesn't support the IPX either (I have Solaris 8 on CD for another machine).