Your horse is really going to need another herd animal as a companion. If you have the ability to be out there with your mare to just hang out while she grazes and that is the ONLY option, then it might work...but then what happens if your schedule changes? You want to go out with friends? Your family decides to go on vacation? Circumstances can change unexpectedly, and depending on how independent your mare is or isn't, it can change her world drastically. High stress can lead to colic and colic can sometimes lead to death. If you at least have a sheep, mini, donkey, llama, alpaca, or really any other herd animal that would be ideal if you can't have another full size horse.
Or as Saskia mentioned, offering pasture "board" for someone's horse, even if they are just paying for feed and time feeding.