Just throwing this out there as an alternate solution...but you could use the
Rhodium Keyboard Controller. Although I don't think you can use its emulate key action, you could use the "send text to active window". Using the "a" key as an example...
From the Keyboard Controller app...
Menu softkey
Add Key
Do NOT check the "Combine with FN key" box
Then press the "a" key on the physical keyboard
Key mode: Single/double click, long press
Event: Click
Action: Default key action [you have to do this in order to get the "a" key to send "a" when pressed normally
Then click on the Event drop-down and select either Double Click or Long Press [your preference although I'd probably go with Long Press]
Action: Send text to active window
Text to send: Press Fn+SYM to pull up the SYM window and find the รก key
Method: Key emulation
This way you could assign one specific special character to the a, e, i, and so on, so that when you press and hold that key for a second or two [if you chose the long press event], or if you double tapped it [if you chose the double click event instead] the special a, e, or i would appear instead.
I'd have recommended you use the Fn+a key, as an example, but you'll run into problems with that. If the default for your keyboard is to insert another symbol with the Fn combination for that key (Fn+o on my keyboard inserts a hyphen for example), you'd loose that default symbol if you remap it. Therefore I'd suggest you either use the key with a double click or long press instead of the regular single click option within the Rhodium Keyboard Controller.