Tom Snow... sorry you haven't gotten some responses on this yet. As you've described your client's professional referral site, I'm assuming that the searches you're targeting are geo-modified names of the professional services that you're selling leads for... eg, doctor, lawyer, plumber, etc... with a given site targeting only one professional niche. I'm not sure how wide a geo area (and I see you've posted another thread asking about optimizing for multiple locations).
They collect leads via fill form then sell them to <niche professionals> in their network
As I'll describe with specifics from Google's guidelines below, this is the important part in relation to schema. I interpret this to say that you are not naming the professionals or their companies on your site... ie, they are anonymous to the user, who reaches them via the form request. This suggests that there's no likely way that your users could be supplying a review per Google's snippet guidelines, as I will describe.
...firms I compete with sometimes use aggregate rating schema, which makes sense for a small business
Yes, it does makes sense for a small business, assuming the page with the snippet could rank for that business by name. It doesn't make sense for a business that you locate via a "professionname place"
search, if there is no businessname identification on that page.
To try to scope this out a bit in the Google guidelines, which, from your questions, it sounds like you already understand...
...aggregate rating schema fall under Google's "Review snippets" guidelines, which one must read carefully to get a sense of Google's intent. Review snippet
The biggest clues fall under Google's "technical guidelines"... and I'll select a few to quote below....
- Make sure to mark up an aggregate evaluation of an item by many people with schema.org/AggregateRating. Google may display aggregate ratings as rich snippets or, for certain types of items, answers in search results.....
- Make sure the reviews and ratings you mark up are readily available to users from the marked-up page. It should be immediately obvious to users that the page has review content.
- Provide review information about a specific item, not about a category or a list of items.
* Recommended: Hesperia Madrid Hotel
* Not recommended: Top 10 hotels in Madrid
And, most important...
If the critic review is for a local business, you must follow these additional guidelines:
- Ratings must be sourced directly from users.
- Don't rely on human editors to create, curate, or compile ratings information for local businesses; instead, use critic review structured data.
- Sites must collect ratings information directly from users and not from other sites.
I don't see how it's possible to follow these guidelines if you're not targeting a specific business-name directly on the page and site on which the review appears, which would rightly be for the small business itself... not for a lead generation site.
I believe I've seen reviews on review sites that effectively sell review space to individual businesses... but users are interacting with the review site, not with the site of the business.
I haven't checked markup in sites like Yelp, but Yelp also is a kind of review directory with comprehensive listings, and I think would satisfy the technical requirements. Yelp is then selling ad space, not business leads.
These are best guesses to respond to your post. Hope this helps.