Fixed loops work fine with sidenocks, provided the loop itself is small enough. If you make them as you would a normal fixed loop it will sit too low in the nock and the forces will act downwards against the lip, instead of around the nock itself.
A running loop works well, but is very tricky to unbraced with particularly heavy bows (130lb and up) in my experience, especially after some shooting when they've bedded themselves in tightly.
There are many other methods of fixing strings that people tend to ignore, and lots can be seen on African and Amazonian type bows, usually without any nocks or grooves at all.
One excellent method is similar to a running loop, but it's folded one more time and pulled through to create an actual knot which sits very tightly into a sidenock but doesn't self-tighten so is easier to unbrace.
Of course, that then poses the question - how were they braced? If using a stringer most methods work but only a small amount of the MR bows had those second grooves. I've seen people brace 150lb bows using just a single knee, a twist of the back or the step through method with ease, and you can very simply lift the loop ABOVE the nock and off the bow entirely doing this. There's no reason to believe the string was kept on the bow as with Victorian style longbows.
If both ends are the fixed knot loop described above, it would be easy to fit one end, bend the bow by hand and fit the other loop over the top of the top nock instead of trying to slide a loop upwards along the limb.
