I think that approach sounds fine. At a minimum, it would require a new subforum where all of the posts could live. There are some tech subforums now that theoretically could be appropriate, but it seems to me it would make more sense to have a dedicated one, to keep everything clearly in one place, as well as avoiding the other forums getting cluttered with these types of threads.
Each thread could link to the machine(s) for which the design/part would work.
The other feature request I would have, would be to add some kind of "machine group" field to the machine database on the site, so when someone designs something that will work for any EM pin, or any Stern Spike 2 pin, or any WPC95 pin, or whatever, the thread's link could be to that machine group instead of having to enumerate every single pin that it might apply to.
This would mean updating the existing database -- something that could be handled by the community, rather than falling on the site maintainer -- so that each machine can be linked to the group or groups to which it belongs. I don't know how database maintenance is handled right now, but presumably there would be a way to view the list of groups, to view a single group's page that shows a list of all the machines in that group, and to see the group memberships for any given machine on a specific machine's page. Membership in a group could be modified either in the group page, by adding a machine, or on the machine page, by adding a group for that machine.
Then a forum search could be performed on a specific machine, and it would return any post that specifically links that machine, or any group that machine belongs to. Note that this feature would benefit across the entire site, not just for this particular application.
I grant that a) the suggestion is still good even without the additional grouping feature, and b) implementation of the grouping feature is likely significantly more effort than just adding a subforum. But it would also improve the site significantly across the board, as well as enable important post-finding abilities.