With all other conditions ideal (no light pollution, no clouds, no moon, right camera gear) but with a high relative humidity (up to 70%), will getting wide angle shots of the Milky Way, for example, be possible?
Ehm, it depends, but more yes than no!
There are some factors that are affected by humidity but most important ones are visibility level and dew. When there is no moon, the former lose its importance. But for sake of explanation, let us assume there is moon and see what humidity will cause:
As the air gets more humid there is more water vapor in the air and this helps scatter the bright moon light, brightening the dark black sky.
You can read more in this Q/A. But to summarize, humidity introduce a little bit of light refraction, resulting in light pollution. The amount of this noise really depends on humidity level and existing light pollution in environment.
The more serious problem however, is the dew. As night goes, depending on the weather and your lens temperature, you quite possibly have to fight with dew forming on the surface of your lens. You can find yourself using a dew heater to evaporate the dew.
To summarize, yes, it is possible to do astrophotography in humidity, specially if the level of light pollution is low. Humidity introduce a pain in the back that is tolerable. The level pain, is a function of temperature, light pollution and other factors. Some good reads:
my two cents: with high humidity, star disks in the photo are horrible! This is an example: http://www.astrobin.com/252070/B/
Picture looks ok but if you zoom you will see how orrible they are. This is not a focusing issue, it appears every time I try to make astrophotography with high humidity (> 80% reported by weather underground)