SOCK_SEQPACKET sockets don't match Racket's port interface, which
doesn't have a notion of message boundaries, so you'd have to work
around that interface mismatch. Also, the unix-socket library reuses
Racket's implementation of ports backed by file descriptors. To support
SOCK_SEQPACKET, it would need a different implementation of read and
write that preserved message boundaries.
So it would be possible, but it's a fair amount of work, and no one
seems to have really needed SOCK_SEQPACKET support yet.
Ryan