Arch Linux

Here is what I found out.

0. The package runs just fine on x86_64 (because the binaries are statically linked)
1. The installer should be put into the noextract= array, to prevent wasting time and disk space (the installer is a self-extracting ZIP file, which bsdtar will detect and extract). Unless whoever takes up maintaining this package wants to copy files without the installer's help.
2. This could be useful: http://www.openwatcom.org/index.php/Unattended_Installation
When using this method of installation, the installer should be run with -dOWDir=/opt/watcom -dDstDir=$pkgdir/opt/watcom -f=options.inf, where options.inf is a file preferably generated from inside PKGBUILD, rather than separately included (to avoid checksums getting in the way of custom builds)

Please add options=(!strip) - the executables contain error messages embedded in debug records.

Interesting to know. I will try once more to see if I can make it compile from source instead.

This is not true. The build.sh script can be used to bootstrap a watcom compiler using gnu c/c++.
This bootstrapped compiler can then be used to compile open watcom.

Also, you could make a package `open_watcom_bin', which is a build dep for open_watcom.
Then package open_watcom provides open_watcom_bin.