Would it be possible to use the pole itself as a vertical antenna?
Yes, it is possible.
I wouldn't worry about conductivity - on the lower bands (80m and 160m) it will
be OK. Most AM broadcast stations use steel tower sections as verticals.
Yes, you'll need a good set of ground radials. How many and how long may be the
topic of further discussion: while "more" and "longer" are usually better, there are
points of diminishing returns. Something like 30 radials each as long as the antenna
is tall is a reasonable starting point.
130' would be a quarter wavelength on 160m, and can be a very effective antenna
(especially in areas with good ground conductivity). Look for the YouTube video of
the 130' vertical antenna going up at RW0CWA. This would also be a half wave
on 80m, but somewhat too long for best results on 40m and higher frequencies.
To use it on 40m ideally you'd keep it below 5/8 wavelength (about 75') though it
can work OK at 3/4 wavelength (about 90'). But with that much height, putting
a horizontal antenna on the top is probably better for 40m rather than using the
mast as an antenna.
You can ground the base of the antenna and shunt feed it, which will effectively
give you a single band antenna. Or you can insulate the base from the ground
and feed it with some sort of matching network: this might be trivial on 160m,
but would need to match a high impedance on 80m. If you want to get more
elaborate you could build a 160m version of the HyGain 18HT vertical with stubs
to provide a match on other bands.
Concrete by itself isn't a particularly good insulator - though it isn't a good conductor,
either. However, if the mast is guyed (as it would need to be to remain upright) then
the base really only needs to keep the bottom from sinking into the ground, and you
can use various sorts of ceramic or plastic insulators that will take the weight of
the mast + the downward force due to the guys when the wind blows against it.
(I've seen shorter masts simply set on a beer bottle as a base insulator.)
One major problem with your proposed approach is that you can't conveniently climb
the mast to add antennas once it is up. One way around that is to weld steps on
to the mast before you put it up. Once you get such a mast in place, you'll probably
want to mount antennas for various bands on it (assuming you've done the engineering
analysis to determine what the safe windload is for it.) Of course, if you have a 100'+
bucket truck handy, that isn't as much of a limitation. You'll also want to secure the
coax feedlines to the tower for strain relief, otherwise the weight of that much coax
may stress the line.
So while such a mast can be a great vertical antenna on 80m and 160m, on the rest
of the HF bands you're best results will be using it to mount other antennas on, which
will require more planning, especially if you are going to add and change antennas