So yeah, question in the title. Does the one handed perk tree have any benefit for a dagger wielding stealth character? Is it worth taking?

Yes, it adds base damage to daggers to pick the perks. Not sure if the standing-power-attack boosting perk will also affect sneakers, I'd assume yes, meaning it'd also be very handy indeed.
Even a small boost in base damage for daggers adds a huge bonus to sneak attack, so it's definitely worth it.
Smithing is also worth using, as it adds base damage to daggers.
Enchanting, however, doesn't add. Even though it says it adds one-handed damage, it doesn't affect daggers.

If you seek maximum sneak-attack, however, you shouldn't even be using a dagger. A huge number of powerful one-handed boosting enchantments will make a one-handed mace a better sneak weapons than a dagger will ever be.

Meh, I'd find it hard to roleplay an Assassin who uses a giant mace. I was just wondering if it was worth investing anything in it. Thank you for the answer.

True, one option is an axe, many of them like the elven are pretty elegant.

Note that if you are an good smith this changes things a bit, the smithing improvements are an flat bonus independent on weapon type and material.
An master smith can give any weapons with material perks an +10 to base damage. taking an ebony dagger from 10 to 20 base damage and an mace from 16 to 26.
This makes the 15*20 and 6*26 look much better for the dagger than the 15*10 against 6*16.

Best of all worlds is to use an dagger left hand and axe right hand.

Dagger is much better weapon for an assassin than a mace.

It has faster swing speed (0.7) and has highest DPS in the game. It's great for backstabbing (15x bonus vs 6x for mace or sword). Dagger is the only silent weapon in the game. If you swing and miss, it does not make noise so you don't get detected. Every other weapon you make noise when you swing it and you get detected.

Disadvantage of Dagger is that it never staggers. Power attack just doubles the damage, but not stagger. It also doesn't benefit from Fortify One-Handed enchantements.

Mace is good because it's good at staggering and does pretty good damage per hit.

Different weapons are good for different styles. There are many situations where a dagger is much better than a mace. There are some situations where a mace has an advantage. For an assassin, a dagger is a much better weapon on the whole.