Forget it - It will not back bud from a stump!
There's probably the biggest reason for this not being possible.
I have seen elms nearly that size dug and converted to bonsai. New shoots can sprout all over the stump which can then be grown into branches and new apex. Creative carving can hollow the stump and make it look almost attractive. That just leave the practical side of things like how to move it, repot it every 3-4 years and the pot size.
This is a pine. Pines do not usually bud on bare wood so unless there is some small live shoots within a foot or so of the ground a pine just won't survive transplant.
Keep talking to the grounds people. They will frequently be altering and replacing plants in the gardens. Knowing the species that transplant well and respond well to bonsai techniques is the key but gardens can be an excellent source of potential trees.
You should be aware of some fundamental things I have found while collecting trees for bonsai:
Trees are always bigger than they seem. Not sure if they actually expand as the soil is removed but every time I have dug something it turned out bigger than I though.
The hole needed to extract a tree will be twice as big as you planned. You will need room to crawl down in the hole to cut those pesky roots under the trunk or to chop almost horizontally with the shovel or ax.
Roots are always bigger than you allowed for so the pot you need will be almost twice what you planned for.
Stumps are far heavier than you imagined. If there's no machine available to lift you should enlist a couple of friends with more muscles than brains.
Collected trees are never as good as they looked before you started digging. Some deteriorate during the actual digging process. Sometimes it is a few weeks before you wonder why you ever bothered.
Digging usually takes far longer than you though and takes much more energy. Roots will be thick so you will need larger tools than you have to cut them. Deeper roots are really difficult to get at let alone cut through. Very occasionally one will just pop out with minimal effort but that is the exception.
Digging trees for bonsai can be lots of fun and some really great trees can be obtained this way. Just pointing out it is not always as simple as it sounds or looks.
