I'm a little worried a solution here will call for whoever controls the webapp to also be an expert at creating placebos for every product type.

How about... company with product type X suggests placebo Y. Webapp/process owner confirms suitability of placebo Y with unaffiliated/blinded subject matter expert in the field of product X. If confirmed as suitable, placebo is produced by unaffiliated external company (who doesn't know what the placebo is intended for, only the formulation of requested items).

Alternately, the webapp/process owner could produce the confirmed placebo, but I'm not sure if this makes sense cost-wise, and also it may open the company up to accusations of corruption, because the webapp/process owner is not blinded to who the recipient company is, and therefore might collude.