It's really easy to train magic resist on any pet in the Enhanced Client; follow these steps for Windows...
1.) Find where your account settings are for each character. By default it's;
C:\Documents and Settings\USER NAME\My Documents\EA Games\Ultima Online Stygian Abyss\User Data\ACCOUNT NAME\SHARD
2.) Right click the characters name and select OPEN and then choose NOTEPAD
3.) Hit CTRL + F keys for the Find option. Type <Macros> and hit return.
4.) On a line beneath there, copy and paste the following into the document;
<UserAction type="Macro" id="25" iconId="8" name="Weaken" repeatEnabled="true" repeatCount="10" binding="">
<UserAction type="CastSpell" id="8" iconId="8" targetType="Current" />
<UserAction type="CursorTargetLast" id="0" iconId="644" targetType="Current" />
<UserAction type="Delay" id="0" iconId="623" targetType="Current" delay="1.96" />
<UserAction type="CastSpell" id="8" iconId="8" targetType="Current" />
<UserAction type="CursorTargetCurrent" id="0" iconId="643" targetType="Current" />
<UserAction type="Delay" id="0" iconId="623" targetType="Current" delay="1.96" />
<UserAction type="CastSpell" id="8" iconId="8" targetType="Current" />
<UserAction type="CursorTargetCurrent" id="0" iconId="643" targetType="Current" />
<UserAction type="Delay" id="0" iconId="623" targetType="Current" delay="1.96" />
<UserAction type="CastSpell" id="8" iconId="8" targetType="Current" />
<UserAction type="CursorTargetCurrent" id="0" iconId="643" targetType="Current" />
<UserAction type="Delay" id="0" iconId="623" targetType="Current" delay="1.96" />
<UserAction type="CastSpell" id="8" iconId="8" targetType="Current" />
<UserAction type="CursorTargetCurrent" id="0" iconId="643" targetType="Current" />
<UserAction type="Delay" id="0" iconId="623" targetType="Current" delay="1.96" />
<UserAction type="CastSpell" id="8" iconId="8" targetType="Current" />
<UserAction type="CursorTargetCurrent" id="0" iconId="643" targetType="Current" />
<UserAction type="Delay" id="0" iconId="623" targetType="Current" delay="1.96" />
<UserAction type="CastSpell" id="8" iconId="8" targetType="Current" />
<UserAction type="CursorTargetCurrent" id="0" iconId="643" targetType="Current" />
<UserAction type="Delay" id="0" iconId="623" targetType="Current" delay="1.96" />
<UserAction type="CastSpell" id="8" iconId="8" targetType="Current" />
<UserAction type="CursorTargetCurrent" id="0" iconId="643" targetType="Current" />
<UserAction type="Delay" id="0" iconId="623" targetType="Current" delay="1.96" />
</UserAction>
5.) Click the Red X top right to close the document. Say YES to saving changes.
6.) Open your UO Account. You should now have a Macro called Weaken in your list. Drag and drop it to a hotbar slot and give assign it a key.
7.) Get back into a guild, and set your pet to All Guard.
8.) Start the macro casting Weaken onto your pet. As soon as you do it will attack. Hammer "All Stop" and then when it does, "All Stay" 2-3 times until it settles down. It'll wander slowly off if you don't.
9.) Now start the Weaken macro again. The
second time it runs it will not attack you, and will sit there being weakened.
10.) If you are getting the error message for too fast casting, right click the macro and edit each of the Delay timers to be a bit longer; it's currently set to recast every 1.96 seconds to match the speed of my own tamers Fast Casting.
You can also clone the macro to get it to run longer and longer by copy and pasting each text block from " <UserAction type="CastSpell" " to " "Current" delay="1.96" /> " again and again. It's much quicker than manually dragging and dropping icons in the EC User Interface
11.) Whilst staying at the keyboard (natch!) every few minutes press the Weaken macro again to keep it running. If you are GM or above Magery, it should take any pet to maximum resist, but the Bane Dragon is capped at GM anyway, so it can be done if you have around 75 magery.
A Mana Regen suit will obviously help too, my tamer is able to keep chain casting permanently without running out of mana.
Casting Weaken will work in the Classic Client too, but I'm not sure what legal process there is for chain casting constantly.