A cap full on 90 weight oil is not going to soften the grease to where it runs past the seal. I’ve been doing this for decades after making sure the bearings do not need adjusting. I fill the metal cap with the 90 weight and hold the cap against the bottom of the hub. Rotate 90 degrees quickly with one hand and tap it in with the hammer in the other hand. How much oil you loose is a function of your dexterity and it helps to do it in the shade of a tree. :B
PS: I put tags on a dozen different types of trailers each year. I also never buy new tires for them as used, US made radial takeoffs (less than 3-4 years old) are better than any of the new foreign made junk.