Shield Master (Combat)
Your mastery of the shield allows you to fight with it without hindrance.
Prerequisites: Improved Shield Bash, Shield Proficiency, Shield Slam, Two-Weapon Fighting, base attack bonus +11.
Benefit: You do not suffer any penalties on attack rolls made with a shield while you are wielding another weapon. Add your shield's enhancement bonus to attacks and damage rolls made with the shield as if it was a weapon enhancement bonus.
(emphasis mine)
So the bashing enchantment is an armour enchantment while merciful is a weapon enchantment. Normally, shields can be enchanted separately as both weapon and armour, so I could have a shield enchanted as a +1 merciful weapon, and a +1 bashing shield, which would allow me to use it as a once enchanted weapon that did the damage of a shield two size categories larger and add the +1d6 of damage from merciful (all non-lethal). I get all that.
But then enter the above feat, that lets me use my shield's defensive enchantment as offense. Great way to save money at the upper levels if I want +5 for both offense and defense, but now what do I have to do if I still want merciful and bashing? Can I pretend that merciful is an armour enchantment, giving me a +5 merciful bashing ghost touch shield for 100,000 gp (equivalent +10 shield)? That seems like cheating since weapon enchantments are more costly than armour ones. Do I have to get a +5 bashing ghost touch shield (equivalent +9 shield) and a +1 merciful weapon enchantment (equivalent +2 weapon)? Semi-related question: if I get ghost touch (the armour enchantment) on my shield, does shield master let me bash incorporeal things with it as well? What about pre-Shield Master feat?
Obviously, the same question could apply to flaming, shocking, or any of the other weapon enchantments that aren't normally shield enchantments. If I put the dancing ability (armour) on it, can it make shield bashes for me like a dancing weapon makes attacks for me (pre or post Shield Master feat)?
Not related, just saw this in passing.
I never ever want a "mercy bashing" by a shield. Sounds to painful.
Per RAW, Shield Master only allow you to use the shield enhancement bonus as a weapon enhancement bonus. As such, allowing an effect of any shield special abilities is at a GMs discretion.
There isn't a dancing armor ability, there is the animated ability, which specifies that it cannot take actions.
I might be inclined to allow the ghost touch shield to have the ghost touch weapon ability (even without the feat), while not based in the rules, it just makes sense to me.