In general I tend to use this
page to as a man-page for 'ash'. But if it comes to some finer details like what options are supported by a particular builtin function of a BusyBox applet I found reading the source code to be the best approach. Of course I would then have to validate any "findings" trough some testing.
My hunch is that the 'readcmd()' function of the 'shell/ash.c' file is responsible for parsing of the 'read' options. Therefore I'd assume that the '-n nchars', '-p prompt', '-t timeout', '-u fd', '-r', and '-s' are supported.