Thanks for testing Maru in the field! I can confirm I've have had the same issue when using hotel TVs too.
I haven't found a good solution for this yet other than adjusting the TV settings. I don't think XFCE itself has overscan adjustments since it's usually done through the graphics card settings app...I know ATI and Nvidia control panels typically have it on traditional desktops. Either way, TV settings is still the best solution because each model may overscan by different amounts.
It still would be nice to have something in Maru to allow manual overscan adjustment for you when you can't access the TV settings. Can you post this as a feature request in https://groups.google.com/forum/#!topic/maru-os/_TL63qbNsKs
so it's on the official list?
For anyone else facing a similar issue, please try adjusting your TV's overscan settings. The actual name of the setting may differ between manufacturers.