Can PETG be used as support material for PLA?
PETG works as support material for PLA, see video

                
            
In theory, PLA printed on top of PETG will be fine because PETG softens and gets sticky at higher temperatures.
Printing PETG support on top of PLA may cause remelting of PLA, but if PETG is kept quite cold (220 °C) the issue will likely be minor. As shown in the video, it works.
PLA/PETG may still be better than using PLA/PLA because of this difference in extrusion temperatures that, for example in bridges where PLA is printed on top of PETG support, should result in very easily removable supports.
The type of supports used should be tested: tree supports could minimize the contact surface between the two and thus minimize marring by the hotter PETG being deposited onto the PLA at the expense of more PETG and normal supports could be used on a limited surface, so they can be removed easily. If the two materials really don't adhere much to each other, you may even be able to fake dissolvable supports, which increase the contact surface but provide a far better finish for bridges and bottom surfaces.
Using a single nozzle may require experimentation. In my experience, I print PLA at 230 °C so it wouldn't be an issue, and 220 °C would also work, but if PLA is printed cold, below 215 °C, you may need to heat/cool the nozzle. In any case, switching filament (especially PETG -&gt; PLA) requires quite some filament to be discarded, so there is time for the heating. It is however to be kept in mind that cleaning of the nozzle between PLA and PETG (or in fact after any material swap) is difficult: some residue can remain in small gaps or low flow areas of the hotend and will be blended into the stream for quite some time after the swap, resulting in hybrid materials of unknown properties.1
As I said, experimentation is needed for this kind of task: my experience, with a 2012-era hot end, may not even be representative of the behaviour of modern hot ends.

1 - this effect is easily noticeable even with similar materials if switching from dark to light color filament, especially if not doing a cold pull to remove most of the material from the melt zone.