74LVC and similar... can a logic 1 output sink current?
It is not allowed, according to the datasheet, to apply any more than Vcc + 0.5V (absolute maximum) to the output. 



However the built in clamp diodes can withstand a certain amount of current (not specified, but below GND -50mA is permitted. 

So with the output high-Z and a pullup to +5 the output will be about at the Vcc rail + 0.7V (or ~5V, whichever is less). The Vcc rail may well not stay at 2.5V and may be pulled up to approaching 5V depending on what else is on the rail, since most voltage regulators will not sink current.

With the output enabled and high, the high-side MOSFET (p-channel) will be "on' and conducting (in reverse) and the above sentence also applies, however there will no longer be a diode drop involved.