I am re-writing some church anthems we used to sing when I was younger and I came across this notation for the Harp. I assume the A is the pedal, but the big question I have is the diagonal black line. Can anyone tell me:
A. what it means
B. how to notate it properly for both look and playback?
I’m not a harpist, just an arranger with some basic understanding of harp technique. My guess is that the ‘A’ indicates not ‘the’ pedal, but how the 7 pedals have to be set to produce a scale of A major.
You may know that a harp has 7 pedals, one for each of the 7 diatonic notes, and each pedal has 3 positions: up = flat, middle = natural, and down = sharp. So, for example, if the A pedal is up, all A strings sound like A flat. In the middle position (the A pedal 1 notch down), all A strings produce an A natural, and with the A pedal 2 notches down, all A’s sound like A#.
A scale of A major is set up by having the pedals (from left to right) at D C# B / E F# G# A, that is: mid-down-mid (for the left foot), and mid-down-down-mid (for the right foot). In modern usage, this setting is notated like
The diagonal stroke in your example looks to me like a glissando that should produce a scale of A major, given this pedalling. But AFAIK, this notation that just gives the key is not entirely standard. I guess a harpist would prefer a pedal diagram like the one above, which Dorico can easily make, or even calculate for you, if you select a musical passage and request to calculate the necessary harp diagram(s) (in the Write menu). Note that you can choose between a diagram or a plain enumeration of pitches by name, in the properties panel below.
I’ve seen it pop up in scores on occasion. If the music is sufficiently tonal it does save a bit of ink, and a player can be expected to deal with it. But IME it’s strongly correlated with lazy part-writing as well so to avoid that impression, better translate it into a full pedal marking and specify the glissando endpoints too. 
