As Vincent has confirmed ... there's no current Postgres support in repserver, but you could ... in theory ... build your own 'module' to do what you want (though don't ask me for the nitty-gritty details).
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At one of my current clients a resourceful developer used the OpenServer (now called SDK) product to build an openserver process that looks (to the repserver's DSI connection) like an ASE dataserver/database.
The openserver process 'just' had to be coded to handle the various T-SQL calls coming from the repserver, parse out the stuff he wanted, ignore the rest, then consume the data as desired.
He then went in and defined custom function strings for the basic rs_insert/rs_delete/rs_update functions for the several dozen tables he was interested in. (From what I've seen it looks like he turns all of the functions into a modified 'INSERT' statement ... which would make his openserver coding/parser a bit easier to write.)
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Worked at a client a couple years ago where one of the development teams used the OpenServer/SDK product to build an interface to allow the repserver DSI to feed into one of the messaging systems (don't recall now if it was Tibco or IBM/MQ).
I know, you're not trying to connect to a messaging system ... the point is that with a bit of coding and ingenuity these guys were able to get repserver to connecto to a non-database platform ... getting repserver to connect to a currently-not-supported database platform is likely doable.
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Several of the current hetero rep solutions have the DSI connect to a ECDA (Enterprise Connecct Data Access) gateway product which then handles ODBC connections to the target RDBMS. This way the repserver only has to know how to communicate with ECDA, while ECDA then connects to the ODBC capable application.
If Postgres has ODBC support then perhaps it may be possible to get a repserver -> ECDA -> Postgres solution to work? [Though you may also have to look at defining some custom datatype translations plus custom function strings.]
You may want to peruse the collection of hetero rep guides to get a better idea of how ECDA works and whether or not this might be an option for you: http://infocenter.sybase.com/help/topic/com.sybase.infocenter.help.rshe.15.7.1.200/doc/html/title.html
Also, the collection of ECDA guides: http://infocenter.sybase.com/help/topic/com.sybase.infocenter.help.ecda.15.7/title.htm
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Other current hetero rep solutions use a custom java-based ExpressConnect module that runs inside of repserver and allow the DSI to connect directly to the target database (eg, Oracle).
Don't know if Sybase/SAP has published the APIs to allow for a custom ExpressConnect module to be used by repserver.
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In Rob's little green book ( Book: The Complete Sybase Replication Server Quick Reference Guide ) he mentions a possible solution where the repserver connects to an ASE as the RDS, where all of the replicate tables are actually proxies that point to an ECDA server that then handles the connection to the follow-on RDBMS (eg, Postgres ?).
Yeah, a good number of components but probably a bit less coding on your part as long as ECDA can handle the connection to Postgres.
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I think you'll find there are a few different ways to do what you want ... just depends on how creative you can get with the skills you have at hand.