Not a perfect solution, buy I think you can use the #+SELECT_TAGS option to set which tress will be exported by default, then tag your desired export tree with that tag. Then when you export the full buffer (not just a subtree) it will choose the tree with that tag.
E.g., --------------------------------- #+SELECT_TAGS: myexporttree * Main tree :myexporttree: fkjaldfk ** subhead ** subhead * anoher heading 1 * another heading level 1 ** subhead ** subhead -------------------------------- In org buffer above it will export only the first subtree now when you export the entire buffer. You should be able to export the others selectively if you choose scope as 'subtree'. And you can always comment out or delete the SELECT_TAGS line if you want to export entire buffer.
