Unless of course you agreed with your lawyers that any invoice would be subject to review by SimpleLegal although I would suspect there would be some opposition to this. Although of course doesn't take away from the value of the service in allowing a company to monitor legal expenditure and to gain greater insight into this (possibly with a view to obtaining leverage for future fee discussions).
On a final point, your privacy policy already assumes that you will use data to improve your service and to provide external reporting. This would seem to imply a degree of aggregation. Although as stated this would be on an anonymous basis, the very fact it happens may be off-putting to certain companies.