I've also been looking into a gauge convertable 830 (DL351 model). A 1.5" scale model could have broad/standard gauge bogies for 7.25" or narrow gauge bogies for 5". Some rough gestimations would suggest that the body would be similarly sized to the 5" 422 from AME, so similar innards could be used. I'd be inclined to drive the bogies via a shaft through the bolster though, as this would simplify bogie removal for gauge change.
Given the relative complexity of manufacturing the bogie sideframes (16 compensating levers ... uuuurgh) I'd plan on having 'swapable' frame stretchers (of different widths) and axel/wheelsets. This would also allow for compliant AALS wheel profiles for the different gauges.
Any contact details for the mob in redfern with the fullsize plans?