v2.5.5 is out. Now you can use built-in search to look for addresses belonging to your wallet and then use a found address to sign any message. Signing and verification is implemented according to BIP322 and is compatible with other wallets who also implement it (I've reused low level BIP322 code from Sparrow wallet, thanks to them for saving me some time). On top of BIP322 I've come up with a "BIP322Share" format to make all signing and verification parts portable and easy to use on mobile devices. Link to BIP322Share description can be found here: https://github.com/akumaigorodski/wallet/blob/master/app/BIP322Share.md, other wallets are welcome to implement it to make signing and verification more accessible.

BIP322 outlines how to sign and verify a message, but what about formatting a result such that it can be presented as QR for example, is there any standard for that? For example, to verify I need to get an address, a signature and a message, so 3 strings. Is there is a format to pack those into something QR-readable/copy-pastable?

v2.5.2 is out. Now you can attach external wallets by providing their BIP39 recovery phrases. Here's a screenshot which shows an example where a single SBW app contains 4 types of wallets including an attached one.