Today I discovered there's an app in Nokia Beta Labs which lets you move maps to the SD card. It's called Storage Check:
You just need to register your Nokia account and phone to the service. With this it should now be possible to offline as many maps as you want on your 620.
For anyone else, the app works with all Nokia WP8 Lumias with the Portico firmware update:
- Lumia 820 or Lumia 920: 1232.5957.1308.0000 and higher
- Lumia 920T: 2041.0009.1308.1002 and higher
- Lumia 520, Lumia 620 or Lumia 720: 1030.6407.1308.0000 and higher)
I'll add a quote:
Lumia Storage Check - Beta (Likely to graduate to a commercial product.)
Lumia Storage Check Official version released as an update via Store,
but Beta 3 stays around.
We have just released the official version as an update via the
Store. It has all the beta 3 features, except the one to store Maps on
SD card. The decision to omit this feature was made after a survey
sent out to many of you. A small, but still volume-wise significant
number of users would be having trouble with the feature and we want
to work on the SD card interface reliability a bit more before
considering the inclusion of this feature again.
As the official version lacks the Maps-on-SD-card feature, we are
going to keep the Beta 3 around for you who want to use it or revert
back to using phone memory instead.