As I am not in Beta - and couldn't comment if I was - the following is my opinion, and not based on fact. Until the final version of SA is released, everyting having to do with SA imbuing will be merely conjecture...
My understanding is that any "artifact" quality item cannot be imbued, and they specifically mentioned invasion weapons in the 300+ post ing about Imbuing. I reread through Post#141 to cite specific example, then stopped

Was time to slay a Shadowlord!
My comment was based on the bow in question being an "event" item, which the Invasion was considered - no? It is obviously an Invasion bow, for it has the same exact three mods every Invasion bow has: HDL, SSI, DI. I have a dozen of the same, regular bows, xbows, and hvy xbows. They all have 25-29 HDL, 35-50% SSI, and 40-55 DI, and all came form the recent Invasion.
re Barry: I too have enhanced Invasion bows, but they were the low end ones, the 30% ssi I enhanced to 40% SSi, but the "General" drops have mods beyond the "norm" loot maximums. You cannot enhance an item beyond its normal loot drop maximum, in this case the bow has 51% (50 max DI on loot drop) and ssi is 45% (max SSI 30% on normal loot drops). The HDL is below the 50% max, however there is no way to enhance HDL anyway...
re Nu: The item does not haveto read "yew" or "valorite" IF it is an Event item or artifact, it is "internally" labelled "made from special Material"...
This is Posting #1 of the aforementioned 300+, and the Official UO reponse. :
"Q: What about certain event items? Will they be able to be imbuded?
Example
Tongue of the Beast (Shield with mods)
Void of Umbra (Sword with no mods)
A: I would guess not, but I don't know with 100% certainty. Most likely, we have flagged them internally as cannot be recycled and made out of special material (like all our artifacts
and the like). If that's the case, then no, they cannot be imbued."