This issue of providing UDP (User Datagram Protocol) support in SOCKLIB has been raised before, and indeed a user submitted a proposed modification to the library for that very purpose. Unfortunately, however, his modifications were to the BB4W version of the library and Windows-specific; it would not have been possible to create a compatible library for BBCSDL. Since compatible versions of SOCKLIB are currently supplied with both BB4W and BBCSDL, it is most important that any UDP extensions preserve this compatibility.
I wonder if anybody would be interested in taking on this task. It would involve examining the UDP APIs provided by Windows (WINSOCK) and SDL (SDL_net) and devising a common interface to the BBC BASIC library that could be implemented using both, whilst preserving as much flexibility as possible. Although I have used UDP I'm not an expert and I would prefer the work to be done by somebody with more of an insight into this protocol and how it is typically used.
Please let me know, either here or privately, if you are interested. If would be a shame if UDP was never supported by the library.
Richard.