You could go with a Farrel or similar 20min base and Burris sinature rings with 20 MOA offset.
He could certainly get to 1000 with that travel, but the trick is getting to 1000 and still being able to get a 100 yd zero. If his 308 load takes 47 MOA (that could probably be improved upon a bit though) and the scope only has 47 MOA of movement, he'd need have it bottomed out perfectly at one end for one range and at the other end for the other range. The odds of this happening with exactly a 20 MOA base are pretty low; it's likely to be off one way or another. The Signature rings could allow for some fudging, but in the end I think it'll save lots of headaches to use a scope with more travel.
I could go further using the mil dots in conjunction with the dial if I could figure the centimeters/hundred meters deal out a little better.
Does the scope have "centimeter clicks?" If so, that's what I and many others greatly prefer with a Mil reticle--they match. "1 Centimeter" click (@100m) is simply a .1 Mil click. 10 clicks from one dot on the reticle to the next.
There's no need to worry about MOA, or convert to meters either. Simply print/record drop data in Mils vs yards instead of MOA. I like that better as well because everything is nicely only two digits long out to way out there:
And those are in yards. So say my scope was extremely limited with less than 20 MOA of "up" (a bit over 5 Mils) available from a 100 yd zero. That would only get me to 850 yds. If I wanted to go farther, I could just continue with the reticle--for 1250yds dial 5.0 ("50 cm") plus 4 mildots down on the reticle would be a total of 9 Mils. Bam! Dead gong.
Of course I prefer to have enough travel, I'm just saying don't get down on your scope because of the Mil clicks. You have a Mil reticle, having both the same is highly useful for many things. If given a chance, you may warm up to it. Another thing that works so well is the wind. Say I call the wind at a nice, even effective 10 MPH at 850 yds. I can dial 10 clicks or I can simply hold one Mil on the reticle. Or at 1250 hold slightly over 1 1/2 on the reticle or simply dial 16 clicks. It doesn't get any easier.