you don't want to have to parse a SLD in your proxy to figure out how many layers it has inside, I believe :-D
Hehe, nope this isn't a good idea and that would also impact performance. My proposal is meant as a temporal solution which assumed that the "fantastic client" will request via HTTP GET. I could also imagine to extent the request limit settings in the WMS configuration of Geoserver (https://docs.geoserver.org/stable/en/user/services/wms/configuration.html) but suppose this is a rarely needed requirement.
@jnh5y for the JTS upgrade for geoserver - it has some failure
geoserver/geoserver#5258
geotools/geotools#3617
GeoWebCache/geowebcache#982
Does this need to be done in conjunction with GT and GWC prs? I think you have a CI integration build showing the combo is successful.