I decided to try 1621+ on my N54L, and it worked well enough, but eSATA drive that I use for backups got recognized as internal. Before that, on 3622xs+, I used these lines in synoinfo:
"internalportcfg": "0x2f",
"esataportcfg": "0x10",
"usbportcfg": "0x1fc0",
I saw some mentions that "you'll have to patch/build device tree manually" but couldn't find any actual information on how to actually do that.