The TYA charge is based on the value of the trust on that date.
If there are costs that have not been paid to that date and have not been recorded as creditors/payable yet, then an accrual can be made. Just include this info as additional info on the D40. Its up to the trustees if they also want to accrue the fee for the IHT return which is usually done after the TYA.
I have always worked on the basis that the figures need to show the ‘true & fair’ value of the trust on the TYA, and as long as the D40 contains the disclosure of any assumptions made than that is fine to include those adjustments.