RV, You're after better heat dissipation capacity. But this solution so far seems fraught with compromise:
A dished front wheel and closer spaced hub flanges will give you a less stable front wheel no winding descents. I have two front wheels with different hub flange spacing, and the difference is marked.
Likely more shudder in the front fork with a disc, especially if you go with 1.125" steerer; that will give you less confidence.
Increased rotational weight, albeit close to the hub
More weight in the fork and disc caliper.
And goodness forbid ravaging your frame to alter in a larger head tube. That sounds scary and beyond consideration.

Just as an alternate thought, consider.
I know you love the bling of your rims, but deep section carbon rims have limited heat dissipation capacity, as any carbon rim is limited in this way. And I don't think you get to leverage the deep section capacity of those rims in your terrain with rolling hills and a speed adverse stoker. If you are looking for better heat capacity, either way you'll be getting a new front wheel, dished, 32 spoke at a minimum for disc loads. But you could raise your heat dissipation capacity all that you need by getting a deep section aluminum rim, 18 or 20 spokes. A rim such as the Kinlin XR 300 or 380, or even a rim by Fir if you go that far. All these rims are well tested on tandem platforms in intensive mountains, even on triplets. The heat dissipation capacity is more than up to the challenge. Carefully select rim tape, which is a weak point on warm rims. One of the couple tandems not to have problems on Ventoux (sp) was an all rim brake tandem. With this set up you get to maintain stable steering, no shudder, and likely less weight than the front disc fork solution.