I've seen a few queries on the PostgreSQL list from people who want to run PostgreSQL on SCO OpenServer, or upgrade ancient versions of Pg to modern ones on SCO OpenServer boxes. Some want to solve issues with connection count limits on their elderly SCO installs, so they can increase client counts above 100 or so. Sometimes people even want to upgrade to a new (ie post-1995, not truly new) SCO OpenServer release and want to know how Pg will cope.
I have a message for those folks, and the management behind them who're usually the ones pushing to stay on SCO.
Your boss may not realize that SCO basically dropped OpenServer as a product line in favour of UnixWare in the late 90s. Since then there was no significant work done on OpenServer. There's been no work done on it at all (as far as I can tell) since Caldera bought the SCO name and OpenServer product from the original Santa Cruz Operation, fired all the software engineers, hired some lawyers and sued world+dog. The Santa Cruz Operation renamed themselves Tarantella after their primary profitable product and went on with life, but "SCO" as a company is history.
OpenServer is dead, dead, dead. Any money put into products targeting openserver is a sunk cost, and you can't change that, but you should really avoid sinking more money into that mess. If your management is still sticking to OpenServer, they should probably read about escalation of commitment, a decision making tendency that's very dangerous and very easy to fall into if you don't think about it carefully.
Upgrading from 5.0.5 / 5.0.7 to 6.0 is like upgrading from Windows 95 to Windows ME in 2010. Or Mac OS 7.1 to Mac OS 9.2. You're upgrading from the corpse of an operating system to one that's still twitching feebly. This is not going to be a good way to invest time and money.
In case you think I'm just a Linux zealot flag-waving, I have a SCO OpenServer 5.0.5 box in the back room, running business critical applications. The apps are actually for Microsoft Xenix (yes, 1983 binaries) running in the Xenix persionality on OpenServer. I considered a port to OpenServer 6.0, but realized it was just slightly delaying the inevitable move to something modern.
So ... I keep it running - in VMWare*, since 5.0.5 runs about ten times faster as a VMWare guest on a Linux host than it does natively on the same hardware. It's faster because SCO doesn't use much RAM for disk cache, doesn't readahead, and is generally just sloooooow in its disk access and memory use strategies. The Linux guest in a vmware setup can cache the whole SCO OS and apps disk in RAM, making the whole setup much faster. It seems more stable under VMWare than running natively on modern hardware, too.
I'd recommend you do much what I've done. Move your SCO instances to VMs running under Linux. Provide modern PostgreSQL on the Linux host, and just compile libpq for the SCO guest. Then start work on migrating your app to run natively on Linux/BSD/Solaris/whatever.
* SCO OpenServer doesn't seem to run under KVM or qemu due to bugs and limitations in their SCSI emulation. VMWare Server is free, so just use that until you're free of SCO.