The idea of dropping the hand on a bag simply for bone density alone is ...well... DVD series !
We hit with bare hands in VT , wall bags with varying density as we progress. We hit a bag in front of us to develop striking ideas along with hip / elbow 'pulses' in a line of force we also use in fighting. If your dropping your arm downwards onto a bag for punching or palm striking work your losing the WHOLE purpose of SLT Dont forget that while punching a wall bag your also traveling a certain way to the bag to ingrain an intercepting line from wu sao to the apex etc...so its multi-tasking along with learning to deliver strikes. Many simply see a bag and want to hit it hard with no ideas of alignment, wu-shin...
Any toughening and bone density is a by-product of our bare hand striking objects unprotected for years.
My hands and forearms from dummy work arent 'normal' but I didnt say to myself one day " I am going to slap my hand down on a bag to make my hand harder " .
I developed open hand strikes that can stop you cold where you stand and drop, but I never once thought to make my bones denser first .
Frankly if you hit a skull with a palm you dont need this 'Iron Palm' you just need good alignment etc..I have hit guys in fights with my plain ol palm and had good results.
I did develop blisters at the base of my palm and then calluses from palm striking work on a wall bag.
Once you get this alignment and line of force you can take it to a heavy bag and start to use turning force/axis, body mass in motion in time with striking entry, etc...
I can also break bricks with my palm. Bricks dont hit back ; )