There's no user demand for running Arrow on Solaris, but CRAN likes packages to build there and at some point we may be asked to support it. This is an umbrella issue for things found along the way.
CI will be another big challenge. rhub does have a check_on_solaris (https://r-hub.github.io/rhub/reference/check_shortcuts.html) but it's unclear if we can call that from a github action. There's also https://github.com/vmactions/solaris-vm and related tools for running a VM on github actions; we can get R from https://files.r-hub.io/opencsw/.