How to write plain Greek in a text cell

Font for the Greek letters is defined in UnicodeFontMapping.tr:

...
0x03B1      N       -2      0x61        # \[Alpha]
0x03B2      N       -2      0x62        # \[Beta]
0x03B3      N       -2      0x67        # \[Gamma]
0x03B4      N       -2      0x64        # \[Delta]
0x03F5      N       2       0x65        # \[Epsilon]
0x03B5      N       -4      0xb6        # \[CurlyEpsilon]
0x03B6      N       -2      0x7a        # \[Zeta]
...

The third column defines the font. You can set it in corresponding lines to 0 (default font).

This approach was used in your link. Difference between the original file and the file in this link:

--- UnicodeFontMapping.tr       2012-10-26 12:11:10.000000000 +0200
+++ UnicodeFontMapping.tr.v8    2013-11-01 22:09:05.000000000 +0100
...
-0x03B1         N               -2              0x61            # \[Alpha]
-0x03B2         N               -2              0x62            # \[Beta]
-0x03B3         N               -2              0x67            # \[Gamma]
...
+0x03B1         N               0               0x61            # \[Alpha]
+0x03B2         N               0               0x62            # \[Beta]
+0x03B3         N               0               0x67            # \[Gamma]
...

For OS X Mathematica v10, one can edit /Applications/Mathematica.app/FontMap.tr to enable writing Greek letters in any font supporting them. In my case, I wanted µ (\[Micro]) to display with my preferred font (CMU Bright Roman) rather than the Mathematica font. The solution was to open FontMap.tr, copy the string within CompressedData[str], paste it into fontMap=Uncompress[str] in a new workbook, delete 181 (character code for \[Micro]) at position 32 of fontMap[[1,2]] by running fontMap[[1,2]]=Drop[fontMap[[1,2]],{32,32}], recompress by running Compress[fontMap], copy the resulting compressed string, past it over the original string within CompressedData[str] of FontMap.tr and then save FontMap.tr. I kept a backup of the original FontMap.tr file. Now Style["µ", FontFamily -> "CMU Bright"] displays µ with my chosen font.

I suppose this could be done for other Greek letters as well.