Seems possible with reference.
"objectId": "[reference(concat('Microsoft.Compute/virtualMachines/', variables('vmName')), '2017-03-30', 'Full').identity.principalId]"
Thanks for the valid suggestion. Your feedback is now open for the user community to upvote & comment on. This allows us to effectively prioritize your request against our existing feature backlog and also gives us insight into the potential impact of implementing the suggested feature.
App Gateway is still very slow :( Even worst with the AzureRM powershell scripts. Takes like 10-15 min to update application gateway. This slows down our CD process.
We are continuously working on enhancing service performance limits. In this occasion we are working on increasing concurrent persistent connection limits.
Thank you for all the votes and feedback. We have started work on this and the capability will be supported soon. If you would like to get in touch with us to discuss your scenarios, please fill this form: https://aka.ms/ApplicationGatewayCohort
Override Backend Path helps a bit in the Http Settings. But we need better rewrite options soon :)