For starters we should choose a wiki (which already has placeholders) and decide on a number of placeholders we want to index. We will then make them indexable and submit them to search engines, so that we can evaluate.
We should start by indexing the first (by id) 3,000 placeholders that are notable (3+ statements, 2+ sitelinks) on cy.wikipedia. Going for the first n placeholders by id is easy to filter (no need to define a huge list of indexable placeholders) and should approximate the importance of the respective placeholders (at least in the lower id ranges).
That should get us started and enable us to see the traffic/ visibility impact of this.