Is this even possible?

[Ultraforce]

This is in regard with a plugin that I am developing. Let us hypothetically assume that I know of a peer that is seeding a file that I want but for some reason the bittorrent client does not find it during the stage of peer discovery. But if I know the IP address and the port on which a peer is seeding the file, is it possible for me to initiate a bittorrent handshake with the client to determine if he installed my plugin? For instance, if the supported messages contains "CUSTOM", then I will know that the user's client installed my plugin. Is this possible programatically?

[Ultima]

There is no difference between a peer manually inserted into the peer list, and a peer returned as part of an automatic discovery mechanism (trackers, DHT, PEX, LPD, etc) other than the fact that the former is not automated. If your extension is reported during handshake, then there's no reason it wouldn't be reported in a manual connect.

[Ultraforce]

For some reason, it doesn't appear to find the peer itself. What I mean by this is that though I am currently seeding the file, my IP still does not appear in the list so I proceeded to manually insert it. I am pretty sure my plugin is buggy at the moment but is there a way to determine the list of messages without using uTorrent? For instance, writing a small handshake client in C that connects to a given peer and returns me with a list of supported messages?

[Ultima]

... Just initiate a normal handshake, parse the replies, and disconnect after the handshake completes. Not sure what else you're looking for.