The idea is to have a accelerando and decelerando markers in the score causing tempo to shift towards a target tempo.

Specification:
Use accel and decel as markers in the score. Use a tempo marker to mark the ending of a accel/decelerando.

| q accel q q q | q q q q | q=200 decel
| q q q q | q q q q | q=60

The tempo shift starts on the note it preceeds and the tempo marker also marks the note it preceeds as it's ending.

| q accel q     q      q |q=150
  |       |     |      |
  |>120   |     |      |>150
          |     |>140
          |>130