Maybe set up a special email address and have it sent out as part of the weekly round-up or something, so that it isn't published on here, only people who get the email.
Set up a spreadsheet with a list of what people want to test, asking them to specify say 3 types of products (e.g. mascara, foundation, hair styling products). Tick when people have tested to ensure everyone gets a go.
Not sure if companies would be bothered sending out to lots of addresses, maybe only to one address. Could it be done via mumsnet towers?
I would love to test organic/natural products, or anything free really