The videos I've seen about the Frame all call out the front expansion port, which "Valve says ... offers a dual 2.5Gbps MIPI camera interface and also supports a one-lane Gen 4 PCIe data port for other peripherals."[1]
That's plenty to support color passthrough as a physical addon, which in turn makes me think that, like with the OLED Deck, we'll see a Frame with built-in color-passthrough later as a different premium SKU when/if they justify it.
I expect that a premium headset is in the works, but they probably didn't want to complicate what is effectively a console launch with multiple SKUs. They'll probably offer a 'Frame Pro' with wider FOV and better cameras a year or two down the line, possibly at the same time as the Steam Deck refresh we all know is coming.
It's conceivable they could opt for better fidelity over more FOV (this would certainly support the 'play 2d games on your headset' push), but I wouldn't put it past Valve to be experimenting with alternative lens designs.
That's plenty to support color passthrough as a physical addon, which in turn makes me think that, like with the OLED Deck, we'll see a Frame with built-in color-passthrough later as a different premium SKU when/if they justify it.
1: https://www.uploadvr.com/valve-steam-frame-official-announce...