Linux

Looks like evremap will do what I want, plus a nifty bonus! The following config lets me use CapsLock + N/P/etc to navigate. And if I just tap CapsLock without pressing anything else, it will act as escape.

device_name = "Telink Wireless Receiver"

[[dual_role]]
input = "KEY_CAPSLOCK"
hold = ["KEY_F19"]
tap = ["KEY_ESC"]

[[remap]]
input = ["KEY_F19", "KEY_N"]
output = ["KEY_DOWN"]

[[remap]]
input = ["KEY_F19", "KEY_P"]
output = ["KEY_UP"]

[[remap]]
input = ["KEY_F19", "KEY_B"]
output = ["KEY_LEFT"]

[[remap]]
input = ["KEY_F19", "KEY_F"]
output = ["KEY_RIGHT"]

[[remap]]
input = ["KEY_F19", "KEY_A"]
output = ["KEY_HOME"]

[[remap]]
input = ["KEY_F19", "KEY_E"]
output = ["KEY_END"]

Note: I used F19 because it doesn't seem to be bound to anything by default. Apparently, a bunch of the other function keys already did things, as described in /usr/share/X11/xkb/symbols/inet

$ cat /usr/share/X11/xkb/symbols/inet | grep FK13
    key    {      [ XF86Tools         ]       };
    key   {       [ XF86MailForward       ]       };
    key   {       [ XF86Word              ]       };      // F2
    key   {       [ XF86MailForward       ]       };      // F3

$ cat /usr/share/X11/xkb/symbols/inet | grep FK20
    key    {      [ XF86AudioMicMute      ]       };