I am currently laying out a board with an STM32F413VGH (UFBGA100) microcontroller. The required pinout is very flexible and only around 50% of the IO pins are in use, which means that is should be possible to fan out the device without microvias. However, annoyingly, both of the VCAP pins are inaccessible without resorting to microvias to drop down a layer. I am wondering if it is valid to just connect each VCAP pin to a GPIO pin (i.e. VCAP_2 to PA10 and VCAP_1 to PB12) to allow them to be escaped to the periphery of the package. I've had a good look around and can't see any evidence of anyone doing this before, but I also can't see any reason why it would be a problem. The core voltage (present on the VCAP pins) will vary between 1.08V and 1.38V and all GPIOs are configured as a floating input on reset. The only potential issue I can imagine is the fact that the GPIO is being driven with an intermediate voltage (in the 'Area not determined' region of Figure 35 of the datasheet), but I expect the Schmitt trigger input to ensure that the pin doesn't enter a metastable state.
Has anyone ever connected a VCAP pin to an (unused) GPIO pin to ease routing?