Can I split a large repair bill on a rental property between current and capital treatment?

Yes. CRA allows a reasonable apportionment of a single large repair bill between current and capital treatment where part of the work genuinely restores the property and part of it improves the property beyond its original condition. You aren't necessarily forced to treat the entire invoice as one category just because it came from a single job or a single contractor.

This comes up often with larger projects that mix genuine repairs with upgrades in the same job, for example fixing damaged sections of a structure while also upgrading materials or adding something that wasn't there before. In that kind of situation, it's reasonable to allocate the portion that simply restores the property as a current expense, and the portion that provides a lasting improvement as a capital expense added to the property's cost. Getting a detailed, itemized invoice from the contractor that breaks down what portion of the work was repair versus upgrade is genuinely the most useful thing you can do here, since a reasonable apportionment needs to be supportable with real numbers rather than an estimated guess after the fact if CRA ever asks about it.