Find the last melody note and identify its pitch (count up the frets on that string from the nut by half-steps, e.g., three frets up from the open G would be Bb). That tells you what key the song is in on the record.
If it's a key like Bb or A where you can use the capo on a lower fret and then play in G, then do so, and there you are. I recommend sticking with such keys where you can do your figuring in the key of G. But if it's C or D you can play along using G tuning and playing the chord progression (when you find it) in the key of C or D. Other keys like E or F would require both a capo *and* playing in D. Remember with any use of the capo, you'll need to tune the fifth string accordingly. But you’ll be on less familiar ground figuring out melodies and solos in those keys.
Once you've found the key, start with the 1, 4, and 5 chords in that key (In G, it’s G, C, D). If you hear what seems like a minor chord, try the 2 minor or the 6 minor (Am or Em in G), the most likely candidates. Using the main melody notes to guide you should work well, but indeed the melody note is not necessarily the name of the chord, but a member. Naturally, not all choices work, but it's a way to narrow it down while letting your ear guide the decision.
Any hints for finding melody notes more rapidly by ear? I know he accented note should be in the chord, but how to tell which one rapidly so that the roll can be modified, etc.?
Find and learn the melody notes *before* you start to combine them with rolls. The more you do that, the better your guessing will get, to where you can make a lot of correct choices on the first guess. Then if your rolling is flexible enough, you will be able to play melody-based breaks more spontaneously.
Is this where more time with scales would be helpful? Should I be spending time with minor or non-G scales for songs which I know aren't in G?
A scale can help to at least identify notes that are likely to be used in the key you're in. No need to practice them for fluency at this stage. I would advise to stick to songs in the key of G, and use what you already know about likely note choices, to keep the job of finding melodies streamlined. I wouldn't venture into finding melodies and soloing in other keys until G opens up for you. One project at a time!
I’ll be getting together soon with as many as three of the other banjo campers. Hope we can keep it productive. Any suggestions will be appreciated.
Play through the group of three-chord songs we recorded on the last day of banjo camp. See if anyone can solo, or at least take breaks consisting of rolls and very limited content. See if you can show your progress on any solo you've been working on. It's a great idea to get together, and four people is an ideal number. Go easy on any songs that have tricky chords, and be sure to have word sheets handy to make it easy on the singers.