Cannot set custom descriptors

Currently I don't see anyway to update the value of a characteristic without using NRF.updateServices which requires that the device is no longer connected to any host

This isn't the case - you can call this just fine when you're connected. It's what it is designed for.

What you're hitting is that you're calling 'setServices' to set up the services, but Espruino can't set them up while you're connected. Once the characteristics are set up though, updateServices will work fine

Also is there a maximum length for the value of a characteristic, in bytes.

It depends on the device, but on Bangle.js the MTU is 131 - I can't remember exactly but I think that means maybe 128 bytes?