I don't know, most magnesium is absorbed by diffusion across the Brush Border Membrane. It seems that in studies at doses above the carrier mediated saturable intake (there are two ways, carrier and diffusion) amino acid magnesium chelates absorb better than magnesium oxide, but not significantly better than food source and slightly less well than magnesium citrate.
If I was pressed to guess, I would say in most instances, the absorption is by diffusion as the carrier mediated pathway is easily saturated.
As for your earlier intravenous question, this would be inadvisable as the magnesium levels in blood are very closely regulated and disruptions may lead to death, although intravenous loading is a (very invasive) way to test deficiency I don't suggest it as a way to get more magnesium.
Supplementing with magnesium citrate works quite well and is safe for all those who have working kidneys.