Step 1: Select an alt character and go into Salon.

Step 2: Select a female/type 2 innerwear that is bound only to the character (i.e. innerwear that is not account-bound) and a female/type 2 basewear that is account-bound (e.g CAST Base F). Can be either Classic or NGS Layering Wear as long as they meet the constraints.

Step 3: Equip a Classic PSO2 type 1/male or All-Race Outfit (e.g. PSO2 Close Quarters, PSO2 Rappy Outfit).

Step 4: Save the Creation Data.

Step 5: Select the character you want to wear male/type 1 innerwear+female/type 2 basewear (different character from step 1).

Step 6: Make sure to have the Outfit used in step 3 available in inventory or storage (anywhere that is accessible while in Salon).

Step 7: Go into the Salon.

Step 8: Select a male/type 1 basewear (Classic or NGS) or male/type 1 setwear.

Step 9: Load the Creation Data from step 4 and remove the Outfit.

Step 10: Review Changes and unselect every box except Basewear/Setwear Selection, Outerwear Selection, and Innerwear Selection. Apply and Save Changes.

Note that you cannot change to another male/type 1 innerwear while wearing a female/type 2 basewear, as they will be greyed out from selection.

To my knowledge, this only works with male/type 1 innerwear, as I haven't been able to trick the game into defaulting to female/type 2 innerwear. Using the same steps seems to default to male/type 1. If you can figure out how, please share in the comments.

As to why you would want to do this? Let's just say, in combination with certain female/type 2 basewears, you can expose certain parts that ordinarily aren't ever visible...