Subject UDFs and what is reasonable

We have a FB 1.56 classic server installation on CentOS 5 Linux 64 bit.  We have developed some UDFs for this that work very well using FreePascal, and are contemplating something that will run as a UDF but will execute code that uses REST to query an external data source (we can control ports, etc. so as not to conflict with other ports in use here).

My question is whether this is outside of the intended limits of what a UDF can do, and whether this will create any issues with synchronous operation of multiple queries on a server.  The server in question has about 100 users, so its very active and although the intended use-case here is isolated to certain specific calls, I'm still wondering what sort of impact this might have on the overall server performance.

Has anything taken UDFs this far and would be willing to share any war stories?