Too late to pollard ornamental cherry?

The main reason for the midsummer recommendation is that, in theory, the weather is dry then. The wounds need a chance to heal over before rain brings in nasties like silver-leaf disease. If there's a good chance of a nice long, dry spell then you could gamble.......
Used to be able to slap some wound sealant on, but the RHS don't recommend using gungy coverings on pruning cuts any more.

 

Fingers crossed