Sanyo Z2: screen size can't be adjusted at 720p

I realise that the Sanyo Z2 is practically an antique now, but I'm still using mine very happily and hope that others are too. Perhaps someone can help me out with this:

I've been experimenting with a Cypress CSC-200 video scaler feeding a 720p signal into my Z2. From what I can see there does seem to be an improvement in picture quality, but it's hard to be sure since the Z2 display setting is stuck on "Full Through": any attempt to change it brings up an onscreen message I haven't encountered before, a rather forbidding exclamation mark in a yellow box.

Sure enough, according to the Z2's manual, the screen size cannot be adjusted with a signal of 720p.

The scaler is switchable between "Normal" and "Wide", but that's not a lot of help for a 4x3 picture.

Is there any way round this, or am I simply going to have to abandon the idea of using the scaler?

Many thanks,

Bert
www.bertcoules.co.uk