Prall in a staff space

[Tyler Mitchell]

I am attempting to put a prall within the staff, in spaces only,
similar to how a tenuto works.

Given the following bit of code:

\version "2.18.2"
\layout { \context { \Staff \remove Time_signature_engraver } }
\relative c' {
  \override Stem.length = #0
  f^\tenuto g^\tenuto f-\prall g-\prall |
  \override Script.Y-offset = #0
  \override Script.font-size = #-3
  f-\prall g-\prall
  \override Script.extra-offset = #'(0 . -0.5)
  f-\prall^\markup {*} g-\prall
}

(The note marked with the asterisk is the desired output.)

How does LilyPond know to place the tenuto within the staff, but
the prall outside of it? And how does it determine that the tenuto
should be placed in a space, but the prall should be placed on a
line?

As seen above, I've managed to use X-offset and extra-offset to
move the prall around (the font-size is just so it fits better) but
I can't figure out how to get it to place it always in the next
available space.

Thanks for any suggestions you may have.

Tyler