From page 5 of 1099-R Instructions:
For a direct rollover of a distribution from a section 401(k) plan, a section 403(b) plan, or a governmental section 457(b) plan to a designated Roth account in the same plan, enter the amount rolled over in box 1, the taxable amount in box 2a, and any basis recovery amount in box 5. Use Code G in box 7.
I think this applies whether pre- or after-tax. Taxable amount in Box 2a would be only the accumulated earnings, if any, on the after-tax contributions.