As is often the case, shortly after posting I figured it out: Select the PGS file then select Yes for “Default Track”.
When I multiplex an srt file it becomes Track 1 on VLC and they play automatically. Can I create an automatic Track 1 with an HDMV PGS file? I tried extracting one and multiplexing a new movie file but that failed. I know I can convert a PGS to an srt.
Thanks.