Sooo... here is what I see...
1) Why do it outside?
If you have 4x4m playspace inside, inside will be better conditions.
Only reason to do it outside is if you placed the PC in the center of the area and attempted to expand the placespace to max walk distance.
I think HTC people have gotten 14mx14m max before the tracking issues become too much. (would have to find the article.)
2) Heat
Additional heart means more sweat.
I am personally tired of sweaty friends moistening my vive hmd.
3) Dirt.
Dragging the equipment through the grass and dirt won't be great and will damage the cables more.
That said.. have fun!