I have tree billboards appearing within the WhiterunWorld cell while running the latest DynDOLOD 3 Alpha 89 (screenshot here to keep my post short). I'm using Perfecter Whiterun, Enhanced Vanilla Trees SE, and Happy Little Trees.
Letting xEdit load my entire mod list and selecting Perfecter Whiterun.esp to "Apply Filter for Conflicts" shows TreePineForest01 and TreePineForest01Dead have object bounds touched by Skyrim.esm, HappyLittleTrees.esp, and Enhanced Vanilla Trees SSE.esp.
Looking at the Worldspace changes, the only other mod with conflict is USSEP for WhiterunWorld, which the Perfecter Whiterun mod page states are intentional. It looks like it's simply DATA Position/Rotation coordinate changes. Looking at each of the placed objects, I can see under XESP - Enable Parent that it is set to PlayerRef (00000014), which seems to match the answer given in the sticky post on Perfecter Whiterun about those flags helping ensure they do not get LOD generated for them.