[Blind Guardian]

Alright, this is how I did it with version 3.0.3, so I don't know if it'll work on later releases.

First, you'll need to make four custom profile fields.

W = Font = Single-Line Text Box

Title: Default Font
Description: Some explaination that this'll make all their posts appear in this font.
Max Length: 100
Display Size: 25
Display Order: Up to you
Field Required: No
Field Editable by User: Yes
Private field: No
Searchable: No
Show on members list: No
Display on: Probably Options, but you could put it on Profile.

X = Color = Single-Line Text Box

As above, just change the title/description.

Y = Size = Single-Line Text Box

Again, as above, but change the max length to 1 (i'm sure you don't want your members posting in size 15 font, this'll make the max 9) and the display size to 5.

Z = Bold/Italic = Multiple-Selection Checkbox

Title: Default Formatting
Description: Blah
Limit Selection: 0
Boxes Per Line: 0
Fields: Bold *Carriage Return* Italic.

Display Order: Blah
Field Required: No
Field Editable by User: Yes
Private field: No
Searchable: No
Show on members list: No
Display on: Same place as the rest.

The letters refer to the field ID, that you should note for each one.

Now, go to the postbit or postbit_legacy template and find "$post[message]"

Replace that with:

<!-- message --> 
	<div id="intelliTxt"> 
		<if condition="!empty($post['fieldW'])"> <font face="$post[fieldW]"> </if>
		<if condition="!empty($post['fieldX'])"> <font color="$post[fieldX]"> </if>
		<if condition="!empty($post['fieldY'])"> <font size="$post[fieldY]"> </if>
		<if condition="$post['fieldZ'] == 1"> <b> </if>
		<if condition="$post['fieldZ'] == 2"> <i> </if>
		<if condition="$post['fieldZ'] == 3"> <b><i> </if>
			$post[message]
		<if condition="$post['fieldZ'] == 3"> </b></i> </if>
		<if condition="$post['fieldZ'] == 2"> </i> </if>
		<if condition="$post['fieldZ'] == 1"> </b> </if>
		<if condition="!empty($post['fieldY'])"> </font> </if>
		<if condition="!empty($post['fieldX'])"> </font> </if>
		<if condition="!empty($post['fieldW'])"> </font> </if>
	</div> 
<!-- / message -->

Replace the letters with the relevant field ID and you should be good to go.