It's a rabbit hole, but there is pretty good support for loading OpenStreetMap data onto Garmin devices (other device makers did a better job obscuring their formats):

http://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin

I guess the routing enabled bike maps will take advantage of whatever data is present.

BBBike supports custom areas:

http://extract.bbbike.org/extract.html

I would buy this if it was significantly cheaper than a phone and made it easy to adjust those tuning parameters. I prefer to keep my phone protected on the bike in case of a collision / fall.