Well, If you are willing to the danger, you can try and see what happens when hotplug is run in the background
not sure if that's going to be pleasant, though
I think there are some other things which IMO are more likely to run correctly in the background, like: loading the hwclock (in rc.S line 185 and 187, but I'm not sure of this one), loading custom sysctl config (rc.S, line 206), cleaning temporary files (rc.S, lines 253-255), setup the motd (rc.S, line 289), initialize /dev/urandom (rc.S, lines 304-314), maybe some things in the SysV init script could be parallelized, setting terminal powersave (rc.M, line 19), saving the contents of dmesg (rc.M, 31), removal of stale locks (rc.M, lines 78-83).
Also, maybe some other things, like running the rc.alsa script, could also be done correctly in the background, but if you run anything else which (for example) depends on alsa running, you'd have to make sure that the dependency has finished starting... that's mainly the reason why I wouldn't rely on having hotplug run in the background.