This is the menu I see when I set that control. What am I doing wrong?
I, er, wow, I didn't think there was a way to bring up the arc-move menu anymore. Is that 7.009? Possibly it's a result of different bindings having curious interactions I didn't anticipate.
Anyway, here's an alternate way of getting straight to Line-Place:
1) On the settings window, click View Controls in the bottom left
2) Switch to the Any Time tab
3) Click the > button to go to page 2
4) On the "Open Line-Place Context Menu" row (either one of them, strangely there's two of them... anyway), click Edit
5) Assign something to the binding; in my example I did Modifier1 = LeftAlt and PrimaryKey = Slash (i.e. forward slash)
6) Click Save and get back out to the game
7) Select your home command station
8 ) SUP Tab
9) Click Mines
10) Press the binding you assigned in step 5
11) The resulting window should have as its top line "Line Place Step 1"