The JAXA G-Portal seems to be queryable, and will produce results that link to https://satpf.jp/spf/ for download. From what I can tell, all PALSAR-2 data has the “Get Data” button disabled for resampled data, and links to https://satpf.jp/spf/ to download the original data, but I have been unable to get this site to load.
If anyone has any insight it would certainly be welcome. It seems like the only option right now is to use Google Earth Engine.