I recently added the HAProxy Wiki page. There is an official OpenWrt package available in the feeds.
I use HAProxy as a simple TCP passthrough SNI based setup for reverse proxy purposes, mainly layer 4, rather than full Layer 7 proxy/TLS termination.
By default the config uses root, which could be potentially an issue with RCE if the HAProxy was ever found to have a vulnerability. I added documentation on how to reduce this risk, given running directly on an OpenWrt router, could allow for accessing a lot of interesting things.