Currently SALT doesn't support this directly. There was an OTN Code project called something like RTMQFORWARD that sort of did the reverse of TMQFORWARD. In other words, it accepted a service request, placed the request on a /Q queue, and then waited for a reply on another /Q queue. Building such a server isn't hard, although this one had an administrative interface and was configurable as to what services it offered, etc. The only major problem with doing something like this is that you may run into timeout issues. You may have to carefully craft the blocking time values to make sure there is enough waiting time to allow requests to complete successfully.
Also, we had considered this use case in the original design of SALT and the Tuxedo Service Metadata Repository, but it was never implemented in SALT. If this would be of high value to you, I would suggest opening an enhancement SR are My Oracle Support to get it considered for a future version of SALT.
Oracle Tuxedo Chief Architect