I think this is the report Rob is refereing to.

https://www.canada.ca/en/health-canada/services/publications/health-risks-safety/residential-radon-mitigation-actions-follow-up-study.html

I agree with the spray foam option. A membrane may be cheaper on a material only point of view but add the labour to get 100% gas tight seal and you will see the foam is likley your best bet.. The foam will provide an excellent soil gas barrier with as close to a perfect (air/soil gas) seal as you can get..

Demilec, as well as other foams have recently been demonstrated through lab testing to perform very well as radon barriers; even outperforming polyethylene. Demilec was one of them. You can contact CUFCA for others. There are actual radon diffusive rates available on the foam that you can use to engineering systems.

Below is a link to a paper on interior spray foam insulation of rubble walls. This is written by Joe Lstiburek of Building Science Press. They will have other reports that will fit your needs if the one I have referenced does not.. Joe Lstiburek is a renowned Building Science Engineer so you shouldn't have any problems getting architects, engineers or code officials to accept his designs. I do not believe he has done anything radon specific but if you apply the spray foam sealing and radon barrier characteristics it will get the job done very well. Remember the lions share or radon entry is mass transport of soil gas and not diffusive transport so a great air barrier is more important than a material with a super low radon diffusion coeffient.

https://buildingscience.com/documents/insights/bsi-041-rubble-foundations

Keep in mind spray foam contains isocyante and should only be sprayed by qualified contractors with proper ventilation and only when the house is vacant. The isocyantes will be used up during the application and will be vented outside. But if you were to spray it indoors without proper procedures and homeowners at home they could be come chemically scensatized. It can be done very safely just pick the best contractor who can demonstrate they know the hazards and control measures. Maybe get an industrial hygienist CIH involved for oversight.

Hope this helps.

I also have some mixed feelings. My fear is a home owner will get a false sense of security or a false low test. I would like to know what meter they are using (professional grade CRM or home owner radon meter). The library should also be concerned about liability. Who is ensuring the calibration and maintenace of these instruments? Are they insured for that?

A neighbour of mine did a DIY SSD and was using a Safety Siren for post mitigation testing. It just kept rising even after multiple resets. My training provided me with the knowledge to question the DIY install and the instrument. The install was actually really well done. He finally let me test his house with co-located RSSI alpha tracks which tested 20 something Bq/m3 beside the meter which was 180 something.

In the end I believe a test is better than no test even with testing inaccuracy. But this program has some flaws.