Sort of got this working on my Amiko 8900 now on the Spark side.
Basically what I've done is to tell it to use the internal flash, but then renamed the directory in the plugins folder and created a soft link (ln -s) over to the real folder which is on the usb drive.
You do need to copy the providers sub folder from the original folder over to the one on the usb.
Haven't tried a download, but it loaded the data that had downloaded on the enigma image fine - I dont see a reason why the download wouldn't work - was just a bit worried about causing problems for the enigma side. Will have a go and see if I can make more progress when I get some time.
Crossepg only gets you so far though - really need something like CoolTV ported over to spark - seeing the schedule for just a single channel isnt as usefull as a full grid view like we're familiar with from $ky/ freesat boxes. CoolTV works pretty well on the enigma side of things - would be nice to see it on spark.
Spark is simpler and more straight forward than enigma - but the lack of a decent epg is it's major failing at the moment for me.