Beneficiary of plan participant disclaimed a portion of the pension benefit to go into charitable remainder unitrust. Does the pension trustee report the distribution to the trust on a 1099-R? The other portion was rolled to IRA in beneficiary name. The beneficiary will receive a 1099 for this part.

Does anyone care to take a stab at this?

This is not my area of expertise. But as far as I'm aware, trusts don't have a blanket exemption from receiving 1099s (like corporations generally do -- with some exceptions).

I agree. That is, I think the trust should get a 1099-R.

The trust must receive the 1099-R for the portion for which the trust is the beneficiary, when that amount is distributed…
This means that the trust was a contingent beneficiary right?...since, as you know, the trust cannot be treated as the beneficiary of the disclaimed portion, unless the trust was also a primary or a contingent beneficiary.
Also, was the beneficiary that rolled over the other portion a spouse beneficiary?...asking because only a spouse beneficiary can rollover inherited retirement assets.