The underground cable from my house to the meter pole (main disconnect)
was installed before I bought this property. They used 4 wire
underground service entrance cable. Two black (hot), one White stripe
(neutral), and one green (ground).

The green wire is connected to the ground buss bar in the house's panel,
but they never connected it to anything in the meter pole main box. The
house has it's own ground rods (2 of them), but I still think that green
wire should be grounded in that meter pole box, not just left
unconnected.

I can see why they did not connect it. There are no spare ground lugs
(screws). I've been meaning to do something with this, and finally went
to do this job after several years. But there is no place to connect it
as far as the lugs. However, there is the thick (#6) bare copper wire
that goes to the ground rod below that panel, and it's right where this
green wire ends inside that box.

My thought is to just put a split bolt around that bare copper wire and
put the stripped end of the green wire into that split bolt too.

But two questions arise. I know this is a GOOD ground, but is this type
of connection allowed by code. Also, the underground cable is aluminum,
and the ground wire is copper. I dont like the idea of dissimilar
metals, since they corrode. However I can get the split bolt that has a
brass divider, that would separate these two metals.

Actually this is the only option I really have, other than just leaving
it unconnected. (which is how it's been for years).