This page attempts to document all the references and nods to UQ Holder in other media. There really is not a whole lot, but the known references are interesting nonetheless.
The Impatient Count and the Time Thief
That's Misleading!
In chapter 2, pages 1-3, a boy with the resemblance of Touta is seen running around, heading to a tall figure in the distance, mistaking it for the Orbital Elevator.
Time Thief
There are more references in the second chapter. One of the main cast is seen reading UQ Holder's premiere issue of Weekly Shonen Magazine. Not only can the UQ Holder cast be seen on the front page of the magazine, but panels of the characters are also visible. It is interesting to note, however, the characters on the panel seem to be Touta and Kuromaru, but the latter does not appear in the first chapter of UQ Holder.
These are seen on pages 6 and 8.
Still Misleading!
The same gag from the previous chapter made it to chapter 3, pages 1-3. The same Touta-like character once again mistook something as the Orbital Elevator and was met with a crueler fate.
Live as if You Were to Die Tomorrow!
There is a small reference on the second and third panel of chapter 7, page 15. These lines are a reference to Touta's shared dream he divulged to Shinobu and the town kids in chapter 2 of UQ Holder.
Wotakoi: Love is Hard for Otaku
A Man of Fine Taste
Tarou picks up volume 15 of UQ Holder while searching for manga in episode 2.