If ever he did get to join, then he should be called a dragon scout, or something.
I can already imagine an episode script if ever this (filly scouts thing) was a big thing. First, the episode will start off with Rarity commending the filly scouts for being so gentlemanly/respectful, etc. Spike witnesses this and makes multiple attempts to try and convince the filly scouts to let him join them, but isn't allowed since he isn't even a filly (plus maybe a couple of other mistakes and things from Spike's end). Then, the filly scouts encounter something they can't handle or solve for some reason. Meanwhile, Spike is on a side quest doing something important for himself. Spike then sees the filly scouts and their problem. Unfortunately Spike's side quest thing is of the same urgency as the filly scouts' predicament, and only one can be done and/or saved, etc. Spike also shows annoyance for the scouts, but their predicament is rather big. As the episode reaches its highest point, Spike contemplates his options frantically. Himself or the filly scouts? Eventually, he decides to put others before himself, and he comes in and helps them out, thus later granting himself a special place within the filly scouts. *Cue awarding of special filly scout badge with the words "dragon scout" in bold caps.*
The episode could even have Twilight popping in at random times within the episode just so that the writers can have her write that letter to Celestia in the end. "Dear princess Celestia, today I learned that no matter what you look like, or what you may be, you can still ..etc etc."
..I just noticed, but this is kinda similar to the storyline of S3E11, when Spike had to take care of all of the mane 6's pets.