You might search long and hard and never find a specific written rule; but as someone who has chaired numerous federal reviews and served as a reviewer as well, a specific line item included in your budget will not fare well with those who score your grant, and it will cost you valuable points! Definitely not something you want to include in such bold fashion.
There are other ways to cover costs. Typically, this is covered as in indirect-cost rate which is negotiated with OMB. If the grant writer and other professionals are hired outside the organization, it may show a line-item for 'professional fees/consultants'. Mind you, the OMB ruling does note that line-items for these things over $5,000 must be awarded by competitive bid, so keep that in mind should you need to use that line item. If you insert professional fees, I would note it to be "costs as necessary to hire outside professionals with expertise not found among staff members, to ensure positive outcomes". (or something like that).
The main trip-up for organizations is thinking they can pay expenses already incurred prior to funding - which is spelled out in several places in your RFP/RFA/NOFA (whatever it is called). OMB circulars are exceedingly specific regarding spending funds on expenses pre-dating receipt of funding.
Hope this helps!