Happy to read about sailfish-compositor plans and poc earlier! In addition to Weston another alternative might be to base the sailfish-compositor on wlroots (https://gitlab.freedesktop.org/wlroots/wlroots/), which is a modular modern Wayland compositor library/framework used by multiple compositors.
I don’t know Weston and wlroots enough to estimate if it’s easier to adapt Weston to sailfishos or make something suitable for sailfishos with wlroots.