Tuning - Most use the (re-entrant) g-C-E-A tuning while some use "key of D friendly" tuning of a-D-F#-B. It's a matter of personal preference. I've thought about both approaches and I don't see much of an advantage with a-D-F#-B, so I'm sticking with GCEA for now. One thing I prefer is a low G for the 4th string. This gives you more range and bass. It's not necessary if all you're doing is playing chords, but I also intend to occasionally play melodies so this tuning will come in handy for that. Most of the banjo uke players I polled were not familiar with the low 4th string method. This is not as surprising as it may seem since the role of banjo uke in oldtime music is rhythmic, driving the backbeat, and it's not necessary to play a single melody note.
Strings - Most everyone said they used regular nylon uke strings, except Jeff Claus uses heavier nylon guitar strings.
Picks - Thankfully, for oldtime banjo uke playing, most players use a very thin nylon pick - .38mm to .60mm. This is good because I had already planned on using a pick to play no matter what.
Banjo Size - most didn't know what size banjo uke they used, although I suspect that it's either soprano or concert scale in most cases. Soprano is a 13" scale, while concert tends to be about 15" from bridge to nut (7.5" from 12th fret to bridge). I'm going with a concert scale of about 14.75".
Rhythm - They all agree that rock solid timing is key. The banjo uke is like a little drum with strings, more percussive than tone, driving the pace. Linda Higginbotham strives to create a seamless, mesmerizing yet energetic sound that gives the music an extra lift. She provides a droning quality by continuously strumming “down-up-down-up” and hitting all the strings each way. You can emphasize the second down strum (the back beat) so it sounds like “down-up-DOWN-up”. If you tap your foot on the beat, every “down-up-DOWN-up should equal one tap of the foot. The tap of the foot coincides with the first “down” while the emphasized “DOWN” occurs while the foot is in the raised position. The idea is to perfect this strum so it sounds effortless.
Chords - Linda Higginbotham also recommends uding open chords (as opposed to bar chords) whenever possible for oldtime music. I've been trying to familiarize myself with open shapes for the I, IV, and V chords in the keys I mostly play in (A, D, G, & C) and the most commonly used minors (this seems to be Am, Bm, Dm & Em. Probably F#minor too!). I also want to learn the 7th form of all the major chords (for example A7, D7, C7). Some uke chords have given me trouble so far - particularly the notorious E and Bminor chords. I've found a power chord way to make an E chord that omits the 3-note (G#) and uses the 1 and the 5 -- two E's and two B's. This seems to work well enough for me. (An alternative is to make the E7 chord which is easier than E). I think I can make a Bminor chord with some practice, or at least a quasi-version of it. One advantage of switching to the A-D-F#-B tuning is if it avoided the chord shapes required to make E and B-minor, but it wouldn't since those would occur for the F and C-minor chords.
Choosing and Substituting Chords - There is always more than one way to chord a tune. The chords chosen should express the overall feel of the tune. I mostly go from fakebooks and tab that already have the chords suggested without deciding for myself. At some point I hope to personalize this technique once my understanding grows. For substitutions, the general rule is that you can substitute a chord for one that it shares 2 notes with. So, you can switch out a G with an Eminor. The G chord contains the notes G-B-D, while the Eminor chord has the notes E-G-B. Both contain the notes G & B. There are other rules of thumb and things you can do like using double stops/dyads, drones, and so on; my approach is definitely going to evolve as I learn more. For oldtime music you don't need to change chords that often, in fact, too much chord changing makes it sound busy and non-traditional. However, I hope to draw from other accompaniment techniques, such as the world of Celtic bouzouki backup, for reference.
Other tips - Stuff a t-shirt, plastic bag or piece of foam behind the dowel stick in the back near the neck to dampen the sound. Strum at different places on the neck and head to vary the sound by moving the strumming arm in a figure 8 pattern - stroke the first “down” on the neck and the second “DOWN” on the head. The “ups” will fall in transitional places as you move back and forth. Finally, carpal tunnel is a concern with this style of repetitive playing. You have to be careful with your strumming and try to avoid using the wrist.