This doesn’t do exactly what you are asking, but if it gets you close enough, you can use the URL hacks to search within a bounding rectangle, or within a radius from a point. See
https://forum.inaturalist.org/t/how-to-use-inaturalists-search-urls-wiki/63#heading--bounding--area
I’m going to guess that generating polygon buffers on the fly may be too taxing on the servers for the developers to consider, but we’ll see what other feedback comes in…