Some more thoughts regarding this...
So after much deliberation I've come to the conclusion that to support the most resolutions and aspect ratios across mobile, tablet and PC without the dreaded black bars appearing I'm going to use a 480 x 320 active game area with a 684 x 384 background. I believe this should cover a lot of devices at integer scales.
Here's a link to a picture showing two examples of how I'm looking to have the game window act. The middle whiteish area is the 3:2 480 x 320 active game area (safe zone). The green window shows a 16:9 640 x 360 area suitable for 1280 x 720 (scale factor 2) and 1920 x 1080 (scale factor 3). The yellow window shows a 4:3 512 x 384 area suitable for 1024 x 768 (scale factor 2) and 2048 x 1536 (scale factor 4).
https://drive.google.com/open?id=0BxC-VOSRkFF-Y0hBaHdOU1Y2WTQ
This setup can also handle 1366 x 768, 1136 x 640 and 1334 x 750 all at scale factor 2 and obviously the safe zone itself integer scales to 960 x 640.
So I guess I need C2 to detect the screen resolution of the device it's been launched on and then scale the game up by the biggest integer that it can so the safe zone is still fully displayed. Seems to me the logic to do this isn't complicated, it just needs to divide the devices x an y resolutions by the safe zone dimensions rounding down to the nearest integer. Then scale up by the smallest of those results, set the displayed area to the devices resolution and centre it on the safe zone. The background would then be in place to fill any spaces that exist around the safe zone.
The only problem now is I'm currently at a loss as to how to implement this...