Buy some land and try to sell the idea to people willing to buy a lineage memorial. To prove out the idea, the question is “who cares enough to pay for it.”

https://www.familysearch.org/ might be relevant for further research.

Wouldn't the target customers of this overwhelmingly be people who chose not to continue their line? Would they care?

Most linage are not linear, they are a messy acyclic linear graph.