> As far as I am aware this approach is basically universal.
You are poorly informed.
> It's impractical to check references before the offer stage,
There's a useful distinction here between a verbal, paperwork-pending sort of offer and a written offer, although seriously, contacting references is not that big a deal. You're talking about a few phone calls. An applicant has every right to assume you've done all the relevant work before extending a written offer, unless you inform them otherwise.
> since the majority of candidates will not want their existing employer to be contacted for a reference until they have accepted the offer.
The majority of candidates are smart enough not to use individuals at their existing employer as references without talking to them about it first. It would be extraordinary for a company to contact someone at the current employer of a potential hire willy-nilly, without permission, as it could put them in an actionable position. Everyone understands the issues involved.
Perhaps you're confusing a reference check with a date of employment check. No HR department is going to hold it against an employee if someone calls them asking to verify that employee's date of employment. It's a procedure that's used for a lot of things, not just hiring.