In general, getting a Feather Rune on a Skeleton Javelineer isn’t straightforward, but it is possible.
Here is one possibility to arrive at the game state I was asking about:
Player A plays and maxes Vandy
Player A plays and maxes Quince, and gets up to 2x Mirror tokens
(some point later in the same game, but continuing from before)
Player B plays Skeleton Javelineer, it has a javelin rune on it
Player B plays Argagarg and summons a Wisp token
(some point later in the same game, but continuing from before)
Player A Mind Controls the Skeleton Javelineer
Player A casts Manufactured Truth on the Skeleton Javelineer they control. It copies a tech 1 unit
The same turn, Player A plays Fairie Dragon, and puts a feather rune on the manufactured truth Skeleton Javelineer that is copying a tech 1 unit
(some point later in the same game, but continuing from before)
Player B plays Skeletal Lord
Player B plays Fairie Dragon (whether this put a rune on anyting doesn’t matter)
(some point later in the same game, but continuing from before)
Player A plays Soul Stones on their Skeleton Javelineer
Player A loses their Fairie Dragon (it dies, etc)
(some point later in the same game, but continuing from before)
Player B is worried that the flying Skeleon Javelineer can do the final points of damage to their base next turn, so they decide to take away its flying for a turn, so…
Player B Polymorphs their own Fairie Dragon
Next turn:
Player A uses Quinces ability to copy the Skeletal Lord with Mirror A
Player A uses Quinces ability to copy the Skeletal Lord with Mirror B
Player A Mind Controls the (Polymorphed) Fairie Dragon
I left out the details of how I arrived at the board state in my previous post, but in general the board state seems ‘legal’ to me.