Are the older (hollow underneath) brick elements considered "collectable"? I know. I know. All LEGO elements are collectable. I mean are the older, tubeless bricks valued above and beyond the fact that they are still viable pieces to build with?
Browsing through the price guides available for the various classic colors of the 1x2 Brick Without Tube part on Bricklink it would appear that the answer is generally yes. The Average US sale price in the last 6 months is about double for the classic 1x2 Brick Without Tube vs the modern 1x2 Brick (with tube) with the exception of Yellow which goes for a bit more then it's modern counterpart.

Avg 6mo Sale Price in US$ for 1x2 Brick Without Tube


Black: US$0.07
Blue: US$0.08
Red: US$0.07
White: US$0.07
Yellow: US$0.08


Avg 6mo Sale Price in US$ for 1x2 Brick (with tube)


Black: US$0.03
Blue: US$0.03
Red: US$0.03
White: US$0.03
Yellow: US$0.02


Do note however that just because a brick does not have tubes doesn't mean that it is absolutely older. The LEGO Group still produces bricks without tubes but generally only in the transparent colors. The transparent colors also tend to become cloudy with time which factors heavily into their price and can be harder to judge from a text listing. It's probably also worth noting that despite the slightly higher prices there are a couple of orders of magnitude fewer sales so older parts may take longer to sell if that is your aim.