I can't see why you would care. Other than the "don't use ports below 1024" privilege rule, you should be able to use any port because your clients should be configurable to talk to any IP address and port!
If they're not, then they haven't been done very well. Go back and do them properly :-)
In other words, run the server at IP address X and port Y then configure clients with that information. Then, if you find you must run a different server on X that conflicts with your Y, just re-configure your server and clients to use a new port. This is true whether your clients are code, or people typing URLs into a browser.
I, like you, wouldn't try to get numbers assigned by IANA since that's supposed to be for services so common that many, many environments will use them (think SSH or FTP or TELNET).
Your network is your network and, if you want your servers on port 1234 (or even the TELNET or FTP ports for that matter), that's your business. Case in point, in our mainframe development area, port 23 is used for the 3270 terminal server which is a vastly different beast to telnet. If you want to telnet to the UNIX side of the mainframe, you use port 1023. That's sometimes annoying if you use telnet clients without specifying port 1023 since it hooks you up to a server that knows nothing of the telnet protocol - we have to break out of the telnet client and do it properly:
telnet big_honking_mainframe_box.com 1023
If you really can't make the client side configurable, pick one in the second range, like 48042, and just use it, declaring that any other software on those boxes (including any added in the future) has to keep out of your way.