ideas?
Are you aware of the Alt method entering a character? It may or may not work with your graphics application, however, it is worth a try. It may be a good idea to try it in Microsoft WordPad first where it is known to work.
For anyone who does not know the technique one needs to do the following.
Make sure that the Num Lock key has been used to switch on Number Lock.
Put the cursor line where the character is needed.
Hold down the Alt key
Using the numbers at the right of the keyboard, key a number.
Release the Alt key.
Provided a suitable number has been chosen for keying, a character will appear provided that there is a glyph for it in the font which is being used.
If the number starts with a 0 or is above 255, then the number is the decimal equivalent of the Unicode hexadecimal code for the character: this can be found using Microsoft Calculator using the View | Scientific mode.
If it is below 256 and does not start with a 0 the number refers to a place in a Microsoft code page. Try 130 and a é character will hopefully appear. Try 142 and a Ä character will hopefully appear.
A few tests will hopefuly determine whether the graphics application works at all with the Alt method and if so, to what extent.
If it does not, it is worth trying keying the text in WordPad and then using copy and paste.
It is good to know that you want to make your own custom font for use with this application.
As you mention that it is a technical drawing, you might like to have a try with one of my fonts to observe how it looks.
http://www.users.globalnet.co.uk/~ngo/GALILEOL.TTF
It is one of a set of fonts.
http://www.users.globalnet.co.uk/~ngo/fonts.htm
Many years ago I devised a scenario to encourage people to learn how to enter words with accented characters in them even if they did not know the language. I called it The Café Äpfel and the idea was that text from ingredients lists from multilingual food packaging could be keyed. The Café Äpfel would have menus in English, French, German and the language of the musicians and singers who were performing in the café that evening. I had this idea of a television show series with each episode combining cookery, computing and music with actors playing the continuing characters and guest musicians and singers arriving as guest stars.
Well, a Portuguese band and singer would be fairly straightforward.
Once the musicians come from further afield the computing gets rather more complicated! 
William Overington