I did some more digging on how to get my Tailscale traffic to bypass a Wireguard VPN which otherwise routes all internet traffic on OpenWrt without using the 'pbr' package, and found a rather strange document:
tl;dr it's definitely possible, but I'm going to have to bite the bullet and not use the simple "route all traffic over Wireguard and enable a 'killswitch' to prevent leaks" two-checkbox setup I've been using thus far.