In Harpers Bazar was a spell 'Holy Weapon' which was similar to 'Holy Symbol' but was cast on weapons and added to OB instead of resistance rolls.
I still use it, with the costs adopted to 'Holy Symbol' of the recent harp edition.
The spell also adds the option to enchant a minor and a major Holy Ability (see customizing the cleric in the core book for examples).
Given that +RR +OB +DB +Maneuver seems to be treated equally in terms of costs by the spell system, that might give you a nice toolkit to develop something without unbalancing the system.
However: adding up to +25 to a weapon which might have additional boni of it's own could be viewed as unbalancing nevertheless - but we're talking 'legendary' here, don't we?
What I like about the Holy * spell is the built-in development: the priest has to enchant it, step by step, while being limited to one scaling option per day, so it'll take time and a bond with the weapon of choice might be developed. The bearer probably wouldn't exchange it easily, even for a potentially better one.
Also it might get rather difficult to successfully cast the highest levels of enchantment:
consider -180 for the major Holy Ability with 36PP used for scaling - not easy even with the necessary 50 ranks in the spell. And the 10 rounds of casting may require a ritual-like setting.
The weapon would be 'holy' only for the caster himself, but that may fit to your ideas anyway.
Of course you may need to construct something for non-spell users ...
Hope it helps despite being only a possible starting point.