well as far as i have understood @BrainSlayer has already ipq806x working in ddwrt, so probably he has not much to answer 
openwrt is a different story (i assume brainslayer's code is not freely available).
i've just replaced my holidays' R7800 with a 5601 (spoiler: no relevant advantages in coverage, the 7800 is still a wonderful beast) and last time i tried it with a nss build VLANs where totally messed up, so i'll give a try to ddwrt just to see if it can really work with recent kernels and DSA, but for what concerns openwrt we probably need to wait..
ok i move my assumption to "i assume it's not easy to port the code to openwrt" 
Nope! There is currently no NSS support for DSA on IPQ806x in DD-WRT.
thanks, missed that branch in discussions
all builds are dsa based since a long time.
Sorry, I meant that you previously mentioned NSS was not yet working with DSA in DD-WRT, based on the post below:
"i have to make a additon. there seem to be indeed a problem with nss in dsa mode if its inuse. luckily i have the nss firmware sourcecodes, but i need some time to get them compiled due the ubicom toolchain which i was not able to get build yet. but i will work on this"