I suppose that you mean layout, not playback - right?

A straight line with an arrowhead is already possible.

Use the Smart Shape Tool’s sub-tool Custom Line Tool (icon: a hexagon).

If I Recall Correctly, you should hold down the CTRL key, and click on the hexagon icon.

You will get to the Smart Line Selection where you can choose, edit or create a custom smart line.

I could be wrong here, but I do not think that Human Playback will understand, that the custom line is a glissando.

As Peter said, the straight line is easy. Could you show us an example of the curved line you want? Does it have to be resizeable?

Thank you both, Peter and Mike.  I didn't mention anything about playback or layout, I don't think. 

The custom line on my version of Finale is an icon of a pen writing a line.  When I select it, however, I can only draw a wavy glissando line with the word glissando written above it.  There seems no option to change that.  

I've compromised by creating a shape in the expression tool.  I have to duplicate it each time I use it, because the exact stretching and angle of the arrow is different each time.  It does the trick more or less.

As for the curvy line, I think I can use an expression for that, because it does not need to be extendable/resizeable.  

The custom line on my version of Finale is an icon of a pen writing a line.

Another needless change, IMHO. What's more, the user manual still shows the snowflake. In any case, CTRL-Click on that icon, and you should get a new dialogue box, showing a lot of custom lines, which you can use as-is, or customize yourself. (Gliss seems to be selected by default.) Choose the line you want, and you should be good from there.

Not sure about the curved arrow, but the straight line seems possible by CTRL+Clicking the Line option of SmartShapes.

I just wrote a new post, only to find that I'd asked about this before.  There is still no solution to the curved arrow, though.  It needs to be basically like a slur in every aspect of its functionality (extendable, attached to noteheads, invertible etc.), except that it has an arrowhead at the righthand end of the slur.  Any more ideas on that topic? 

Thank you for the previously replies.  It's strange that there's no other way of opening that dialogue box other than holding down control and tapping on the custom line button.  How would one know about that shortcut?  And how would one be able to find this dialogue box without the shortcut?  I search the manual for arrows and came up with nothing.

The Guitar Bend tool seems to do most of what you want. It lacks the extra control points of a slur, but it may be better than nothing.