Adding a "custom preset" and accessing it via a "custom GCode placeholder"

Hi,

Is there a way to:
1. Add a new parameter (aka "preset") - (Maybe via the PresetEditor.pm? i.e. I prefer using Perl if possible to avoid rebuild any Slic3r binaries)
2. Access the value of this new preset from a Custom GCode placeholder?

For example,
Let's say it I want a completely new 'int' value (e.g.: "special_delay") to be set and configurable by the user, like any other Slic3r setting.
And I also want this "special_delay" to be accessible via the Custom GCode module.

e.g.
I should be able to write a Custom GCode line like this:
G4 P{[special_delay] + 20} ; do special delay

EDIT1:
To hopefully make my example a little more realistic:

I am using multiple extruders, and I need my Hardware to make a small delay on every "Tool Change".
Tool Change events are supported by Custom G-code hook, so I decided it would make sense
to implement my delay via the Custom G-code mechanism, like that:

To here:
Printer Settings -> Custom G-code -> Tool change G-code

Add this line:
G4 P1000 ; wait 1000 ms here

Well, so far so good: The G4 P1000 line will be injected into the generated GCode whenever a tool-change occurs.

But now I wish to make this delay a parameter (aka "preset", I'll name it 'special_delay' as before)
Allowing the user to normally change its value via the Print Settings section, and save this value.
(Just like he/she would change and save the Print's "Layer Height" in mm, for instance)
And, this new 'special_delay' parameter should also be visible to the Custom G-code parser.

And I'd expect the "Custom Tool change" line I'll write should become something like this:
G4 P{[special_delay]} ; do special delay according to user 'special_delay'

The Custom G-code parser is expected to replace the placeholder [special_delay] with the actual value set by the user.
So, assuming the user changed the value to 1500, the injected G-code line would be:
G4 P1500 ; do special delay according to user 'special_delay'

Hope this edit makes my question clearer... (-:

Or, maybe, this is the wrong approach, and I should prefer the GCode post-processing for this task??

Many Thanks!!

Edited 2 time(s). Last edit at 06/25/2019 02:15AM by ShaharB.

I'm using a post-processing script to do something similar, but the user has to set the value at the top of the custom g-code. For example...

G-code:

;define placeholder values here
;>>[custom_special_delay]=1000

;use a placeholder anywhere after its definition
G4 P[custom_special_delay]

Post-processing (JavaScript using the Windows Script Host):
replace_custom_placeholders.cmd

@if (@CodeSection == @Batch) @then
  @cscript //Nologo //E:jscript "%~f0" %* & goto :eof
@end

//[stackoverflow.com]
//[docs.microsoft.com]

var FOR_READING = 1, FOR_WRITING = 2;

//get the contents of the file
var filename = WScript.Arguments(0),
	fso = new ActiveXObject("Scripting.FileSystemObject"),
	file = fso.GetFile(filename),
	ts = file.OpenAsTextStream(FOR_READING),
	input = ts.ReadAll();

ts.Close();
ts = file.OpenAsTextStream(FOR_WRITING);

var rxp = /;\s*>>[(custom_\w+)\]\s*=\s*([a-z0-9]+(?:\s+[a-z0-9]+)*)\s*(?=;|\r?\n|$)/i,
	match;

//overwrite the file as placeholders are replaced
while( (match = rxp.exec(input)) ){
	//WScript.Echo("Definition found:", match[1], match[2]);
	//write/append the text before-and-including the definition
	ts.Write(input.slice(0, match.index + match[0].length));
	//truncate the input, and replace the placeholder throughout the remainder
	input = input.slice(match.index + match[0].length).replace(new RegExp("\["+match[1]+"\\]", "ig"), match[2]);
}

//write/append anything left from the input
ts.Write(input);
ts.Close();

Many thanks Wizard04 for your detailed answer.

I guess doing it via Post Processing is the fallback I am finally going to find myself implementing.

I am building a customized printer for a customer and preferred to make it as user friendly as possible and as less error-prone as possible.
The most straightforward way I could think of was adding a parameter ("preset") to Slic3r.
But apparently, as implied from the lack of replies, it is not such a straightforward way after all...

I'll probably end up creating some simple GUI (just for entering and validating parameter values) behind post processing.

I'll update.