[2019] Sundial weapons can be used for infusion without losing the weapon.

Complete a sundial run, claim your weapon, without leaving the reward selection screen open your menu, inspect the weapon you want to infuse, there will be 2 of the sundial weapon available (one with perks and one without), use the one without perks for infusion, you will keep the original and your infused weapon will keep it's new LL.

Edit: with further testing I tried with primaries full. I chose the sidearm. It went to post master, but when infusing I still had a sidearm with no perks of to use.