All I can add is to say you'd definitely need a rear derailleur or tensioner as the chain would be slack when on the small chainring. With that in mind it'd probably be a more elegant idea to lose the planetary hub and go for full rear derailleur as the chainline will be an issue going from front derailleur to fixed-position rear sprocket and may be droppy. Also - a tensioner will add weight so might as well keep it all to one system.
From what I've read about other twentys with rear cassettes, it's hard to get clearance for the smallest sprocket though and you may have to forfeit the highest speedy gear, though my hunch is with the right parts, a dished wheel and the optimum spacing along the axle and this may be preventable.
Third thought if you like routing cables is to pick up a SRAM dual drive - hub and derailleur and front double = 2.43 trillion usable gears.
