|Weevil mine in Viscum|
While looking for this species last year we did establish that a weevil, most likely Ixapion variegatum which normally feeds in the stems, can on occasion mine a leaf. The larvae are creamy white, c shaped and lethargic compared to woodiana larvae which are green. My photo shows such a mine. In this one the initial frass filled track has been absorbed into the developing blotch. Looking at your photos, I wonder if your mine might belong to one of these weevils although it is hard to tell from the photo and I am certainly not sure. If the larva tends to appear c shaped and pale I suspect that is what it is. If it is mobile and not permanently curled up then you probably have woodiana. I would not be surprised if early instars are not the bright green they end up being later on.
The moths we bred last year were from much more mature mines found in early May, at which time they are near full fed. I suspect keeping such an early one alive would be a challenge although you can keep mistletoe looking reasonably fresh for some time standing it in water.
One thing to do is revisit the trees a little later in the season, say from mid April, when woodiana mines will be much larger and the larvae easy to confirm. In the meantime you might give Tony a shout let him have a look at your mine.