Alibaba advertises a 156mm x 156mm solar cell with 4.38 watt output. The space on Minitec enclosure for membrane keypad is 34.7mm x 19.6mm, or 680 mm2. The solar cell is 24336 mm2. 24336 / 680=35.8. 4.38 watts / 35.8 = about 120mW.
This LiPo battery is 160 mAh * 3.7V = 0.592 watt-hours. 0.592 watt-hours / 0.120 watts = 4.9 hours to charge the battery using a piece of the advertised solar cell in direct sun at the equator at if it’s cool. It won’t work out exactly like that but at least it’s a possibility.
The iPod nano 6 LCD is about 32 x 33, or 1070 mm2. If it was replaced with a solar cell the output might be around 0.2 watts.
I still think the OKW Minitec is best solution we have available to us now. It would be interesting to see how much power a solar cell on the front of it would provide in normal use (worn on the wrist with a short-sleeved shirt on a bright day with all the window coverings open).