All I know is that, if the cutoff for Eroica is the 1980s, there were already several bike computers on the market.  I myself got a terrible product known as the CatEye Solar that would never stay charged up even when parked in the sun all day.  Also, I came across an eBay listing that suggests that the old Avocet speedometers are that vintage. 

I just searched using google, for "bicycle "computers" eligible for eroica"

What I think might be even more fun would be one of those mechanical speedometers with a spinning cable running from the hub to the display unit, or even the odometer mounted on a fork blade that worked by counting the number of times that a tiny pin (mounted on a spoke) hit a toothed wheel.  Perhaps those are old enough to be considered valued antiques.

Good luck with this.  I'm all in favor of anything that keeps useful (to someone) bike things out of the landfill while providing enjoyment to bicycle enthusiasts.

Gordon