there was a similar topic some time ago you can check.
But, generally, right way will be using internal scripting coming in 1.50 and use script command to pass url to it.
You want to open hippoedit as single instance passing url to it to be open in internal browser?
Thanks for the link. Yes, I want to open the file which has the same name as the current one and is located in the same folder but has another extension (html instead of textile) and which was created by another tool. I'd like to open it in another tab of the same instance of HE (which is easy to do for the html source code).
BTW: if you will publish you syntax schema for textile you will get a free license 
From another side I am waiting for one more bug reported from you. It will be also enough, but with a new syntax schema definitely 
Of course , if you already do not have one.
Great!

(No, I don't have a licence still.)
As for publishing textile scheme, I plan to do so, but I'd like to polish it a bit first.
(Not all textile features are implemented so far even where HE would make it possible and colors of styles are hard-coded, not using colors from palette so far. Morover, it's not a classical syntax highliting in a sense, but a highlighting making a sort of WYSIWYM textile editor from HE (e.g., not just "tags" indicating italics are highlighted, but all the text between them is shown in italics). (But perhaps, that would be not a problem...)
Another issue is, my scheme doesn't make a proper use of inheritance so far, I'm afraid, as I don't understand the working of the feature properly so far...)
So, I have to improve it first a bit, definitelly...)
EDIT: Moreover, I should write a "civilized" comments on "special" features of the highlighting and on its shortcomings due to HE's syntax definition limitations.