If you're looking at proxies for their contents (you're ignoring the fact that it's a proxy and just treating it like a message), then you already have to deal with the fact that you can't reply to a proxy under QNX 4. Under QNX Neutrino, you can't reply to a pulse. What this means is, you've already got code in place that either looks at the proxy ID returned by the receive function and determines that it shouldn't reply, or the proxy has encoded within it special indications that this is a message that shouldn't be replied to.
Unfortunately under QNX Neutrino, you can't stuff arbitrary data into a pulse. A pulse has a well-defined structure, and there's just no getting around that fact. A clever solution would be to “simulate” the message that you'd ordinarily receive from the proxy by using a pulse with a table. The table would contain the equivalent messages that would have been sent by the proxies. When a pulse arrives, you'd use the value field in the pulse as an index into this table and “pretend” that the given proxy message had arrived.