I use this stance sometimes while doing Small Universe. I find it very effective to just hold it while circulating through the energy centers.
It does not seem to be counter to either the purpose of the meditation nor Chunyi Lin's instructions: "You can do this exercise while sitting, standing or lying down."
Also, in the Level II course he points out that the "mudra" (hand position) one holds while meditating is not that critical, though he gives the open palms and the "holding the world in the palms" (what I call it. It is the mudra one uses in Zazen the way I was taught it) as examples.
For me, holding the tree (or holding the energy ball) are mere hand positions and being tried and true in qigong tradition, they do not seem to have any counter indications while doing Small Universe.
I do not do a very deep or wide horse stance and never have, even when practicing in martial arts. The forms I learned rarely used wide horse stance for standing exercises.
