Well, I believe there is.
A button could open a link. This link opens a PHP script on the same server. This PHP script writes a file which is read through AMXx probably by intervals and AMXx uses the information for further action.
It's an extremely shitty way, especially the file read. May be possible using sockets instead but I doubt it. If you know when to search for the php generated file there can be improvements instead of constantly checking for files.
Unless there's a good reason I won't spend much time researching.
And a webserver with PHP on the same machine as the HL server is a requirement.
Basically, it can be used on your server but not for a public release of a plugin.