Currently go-app assumes that the target architecture for the client side must be GOOS=js GOARCH=wasm. Here is a POC of the Lofimusic app build with GopherJS with a minor patch to the go-app:
https://github.com/nevkontakte/lofimusic/tree/0d328bf50b6fe7a3fc5f1a5d0d06a99f3e7cc368

The compiled code for GopherJS looks very different from Go-wasm, but it supports all the same APIs, including "syscall/js". The only major difference between Go-wasm and GopherJS is how the code is initialized: GopherJS produces a self-sufficient JS script, so it only needs to be included in the page via a <script> tag.

If as a go-app user you want to make a choice at the build time whether you want to use GopherJS or wasm, the framework could generate the appropriate initialization scripts.

As an aside, although GopherJS does support syscall/js API, it is a wrapper around the github.com/gopherjs/gopherjs/js package and is a bit slower than using the latter directly. It should be easy to make a GopherJS-specific implementation of that for GOOS=js GOARCH=ecmascrit target, thus gaining some performance.