The ornamental design for a treemountablehoistsupport, as shown and described.
FIG. 1 is a top right isometric view of a tree-mountable hoist support, showing my new design;
FIG. 2 is a left side elevational view thereof, the right being a mirror image;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view.
The dash-dot broken lines shown in the figures are included for the purpose of illustrating portions of the tree mountable hoist support and form no part of the claimed design.