I own a RISA solid soprano. Love it!
Mine has the friction tuners, and while I don;t at all mind friction tuners in general (two other of my ukes also have them), they take a lot of work to turn on the RISA. Ouch! Most key, though, on mine--and this is my only real knock--is that turning the tuner does NOT always change the pitch of the string. It sticks, if you will, at the metal saddle rod. I must pull the string each time to get it to go, or else there's a whale of a tension on the string. Hard to explain, but a chronic issue with these. The snapping "A" string is well documented on UU, and I have snapped one myself. So, although I don't know how to get the knot out of the hole, I wanted to make you aware of the snapping A string syndrome on these, and I commisurate with you.
Other end of the spectrum, as you might be tempted to make the knot smaller so it wont get stuck again, but remember, the knot will have to be big, or else eventually it will work its way through the neck completely (as happens sometimes to those with through-body ukes). Then, it'd be a neck repair (or tying a giant knot!). I do not like the way the strings are on the head of the RISA. Some ukes have through body design on the saddle end, but there a large knot doesn't interfere with playing--it's inside the body. Or, as ukeeku.com once told me, people can use the round circles cut from the end of old guitar strings (the little metal donuts) to tie the nylon uke string to so it will not eat through the body. Well, if we did that excellent idea on the RISA, there'd be no stuck string, but we'd be fingering over the donuts at the head end, if you get my drift. It wouldn't be flat anymore.
I'd use a fine wire, and perhaps a quick spray of silicone or WD40 first on the knot end to free it up. I'm certain you can get it out--it;s just how to handle the next install that will predict the future for issues. Please do post when you get it out, how you did it, and how you restrung it. CK1
Martin Sopranos: 0M: 30, 64 --- 1M: 22Dit, 23Dit, 49, 55
2M: 22, 23Dit, 49, 55, 60, 12 --- 2K: 25, 12
3M: 25, 48, 12(c)
Martin Tenor & Concert 1M 40
The Gibsons: Styles 1&2&3 28; Gretsch American 30
Asturias Japan SuperSop/Tonks 28,30/Favilla 55
Mya-Moe 851 WalRed Bari/1141 Syc Sop
Ko'olau CE-1: Koa Tenor & Myrtle Tenor/Koa T100
Pono Maple Tenor & MS Sop