First-off: I find your current technique for CIH to be very cool.
And it's a pity that a feature that's so fundamentally simple isn't implemented on the BenQ; I'm guessing most people don't think to use this technique.
Onto your question: I don't think the overscan feature will give you enough range to do what you're after (it only has 3 steps on the W1070; I'm sure the W1300 is the same); but Digital Zoom should be sufficient (much more range).
But I have a feeling that the simplest solution in your case (ie, replacing manual changes to zoom with a digital equivalent to achieve CIH at the expense of cropping) will actually be just to use the projector's built-in 'Aspect Ratio' function to select Letter-Box, which does the zoom/crop automatically with one button.
And while there's definitely a hit on quality in losing the 1:1 mapping, I've found that I don't notice all that much resolution loss as long as the source quality is high-bitrate - such as a blu-ray.
BenQ HT2050 Projector; Kodi (on Ubuntu, OpenELEC & S802) via Sony STR-DH540 AVR with Boston Acoustics SoundWare XS SE 5.1 Audio + Jamo 210 Sub. Emby for Mobile Streaming.
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