Mechanical-wise it is OK, but thermal is a concern if mounting the BBU on the add-on card.
Please add BTR-0018L-kit (to convert BBU into 2.5” HDD form factor) and move the BBU to the front. You may use either one of the hot-swap HDD tray (by adding 2.5”HDD convertor tray MCP-220-00043-0N). Or add an internal 2.5” HDD bracket (MCP-220-82603-0N) and secure the BBU on the bracket.