Does Mairsil, the Pretender ability work on activated abilities that are not reliant on being on the battlefield? Or is his second ability only in effect while he-himself is on the field?
For instance Yuriko, the Tiger's Shadow activated keyword Commander ninjutsu? Or Reassembling Skeleton? (If either is a bad example please use a better example of an activated ability usable from another zone, hand, graveyard, etc.)
Unless otherwise specified, static abilities like the second ability of Mairsil, the Pretender function only while the card is on the battlefield.
604.2. Static abilities create continuous effects, some of which are prevention effects or replacement effects. These effects are active as long as the permanent with the ability remains on the battlefield and has the ability, or as long as the object with the ability remains in the appropriate zone, as described in rule 112.6.
112.6. Abilities of an instant or sorcery spell usually function only while that object is on the stack. Abilities of all other objects usually function only while that object is on the battlefield. The exceptions are as follows: [...]
To clear up some confusion: All abilities that aren't spell abilities, activated abilities or triggered abilities are static abilities and are subject to the rules I mentioned above. But there are some exceptions to rule 112.6 that define which abilities might be effective even outside the battlefield. Some examples are:
