Feature request: option to have overload default to starting key + pressed key

[firtrees]

I'll start by thanking the developers for this tool, which I find to be very useful.
The rationale behind this request is similar to that of #651, in that I have a working configuration which is however verbose.
I would like to overload the space bar, which usually comes with some typing limitations; the workaround I am trying out is the following: while holding space, bound keys should work as instructed, while pressing an unbound key like a should produce the sequence space a.
This configuration nearly works as described:

# Overload space to produce sequences reminiscent of common terminal shortcuts
space = overload(terminallike, space)

	[terminallike]
	h = backspace
	j = enter
	k = tab 
		# the terminal uses "i" for that
		# but, unlike "i", "k" is not used much at the start of words
	l = delete
	[ = esc


	a = macro(space a)
	b = macro(space b)
	c = macro(space c)
	d = macro(space d)
	e = macro(space e)
	f = macro(space f)
	g = macro(space g)
#	h = macro(space h)
	i = macro(space i)
#	j = macro(space j)
#	k = macro(space k)
#	l = macro(space l)
	m = macro(space m)
	n = macro(space n)
	o = macro(space o)
	p = macro(space p)
	q = macro(space q)
	r = macro(space r)
	s = macro(space s)
	t = macro(space t)
	u = macro(space u)
	v = macro(space v)
	w = macro(space w)
	x = macro(space x)
	y = macro(space y)
	z = macro(space z)
	

	0 = macro(space 0)
	1 = macro(space 1)
	2 = macro(space 2)
	3 = macro(space 3)
	4 = macro(space 4)
	5 = macro(space 5)
	6 = macro(space 6)
	7 = macro(space 7)
	8 = macro(space 8)
	9 = macro(space 9)

	
	- = macro(space -)
	= = macro(space =)

I think it would be useful to add the possibility to have a layer activated by a key X (whether held or oneshot) to produce macro(X a) when an unbound key a is pressed.
An idea could be to allow for something like [layer:X] as a layer name (which would however cause problems if one wants to send both keys X and Y to the same layer), or maybe [layer:pass] (or something along these lines) to toggle this "passthrough" behaviour of the key.

I am unsure whether a pressed key should be considered released after this behaviour has occurred once: in my use case this scenario just doesn't happen, since I'm simply looking for a way to deal with the sequence <X on> <a on> <X off> <a off> (which admittedly looks like a common problems people seem to face), but I cannot imagine how people would intentionally exploit the feature by keeping X pressed.

Thank you for your time!