Sussed out the watched folders thing.
When you export the J River Registry it ends up with the watched folder part in this format:
<XMLPH version="1.0">
<Item Name="Ignore Special">1</Item>
<Item Name="File Types">m4a,aac,3ga,ac3,aif,aifc,aiff,au,snd,aa,caf,cue,dff,dsf,sacd,dts,flac,fla,m4p,m4b,mka,mid,midi,rim,rmid,ape,apl,mac,x-ape,mp4,mpc,mp+,x-musepack,mp3,mpa,mpga,jmx,ogg,opus,ram,ra,shn,tta,wav,w64,raw,l16,wv,wma,wax,x-ms-wma</Item>
<Item Name="Folder">\\server\Music\iTunes\iTunes Music\Classical\</Item>
<Item Name="Tags">(1:1)(1:0)</Item>
</XMLPH>
But you can't import that as a single REG_SZ string - so you have to edit it to look like below.
"Watch 0"="<XMLPH version=\"1.0\"><Item Name=\"Ignore Special\">1</Item><Item Name=\"File Types\">m4a,aac,3ga,ac3,aif,aifc,aiff,au,snd,aa,caf,cue,dff,dsf,sacd,dts,flac,fla,m4p,m4b,mka,mid,midi,rim,rmid,ape,apl,mac,x-ape,mp4,mpc,mp+,x-musepack,mp3,mpa,mpga,jmx,ogg,opus,ram,ra,shn,tta,wav,w64,raw,l16,wv,wma,wax,x-ms-wma</Item><Item Name=\"Folder\">\\\\server\\Music\\iTunes\\iTunes Music\\Classical\\</Item><Item Name=\"Tags\">(1:1)(1:0)</Item></XMLPH>"