Michael Beijer 

Without testing it I would guess at this:

Michael Beijer said:

Does the XML file need to be closed?

Yes.

Michael Beijer said:

Do I need to restart Trados Studio?

No.

Michael Beijer said:

Does it work better or worse with Lookup enabled? (Options > Editor > Automation > Enable LookAhead)

Lookahead will always cause problems for you if you are expecting changes to a result in a recently cached segment.

Michael Beijer said:

For example, one idea I had was to create a small AutoHotkey script which would allow me to quickly add term pairs to the XML file via a small GUI/shortcut.

It's a good idea... but could be problematic without a refresh somewhere.  I guess... if you're prepared to validate this stuff we can take a look and add an enhancement to improve this feature in the future.

Thanks for the info Paul!

Incidentally, if I disable LookAhead (via Options > Editor > Automation > Enable LookAhead), will this immediately work in my current project? Or do I need to create a new project with it already disabled?

Indeed, it's needing to refresh somewhere that I am wondering about. I'll have a fiddle and report back here. An enhancement to improve this feature would be very welcome, as I think it's a killer feature (if implemented right).

Google Translate + GT4T's ability to quickly add custom terms = a very powerful combo when translating patents in my daily work.

Wow, OK, so I am currently testing it. I find that pre-lookup find/replace sometimes causes strange issues, so I'm using post-lookup find/replace, which is actually working amazingly well. And I don't even need to close the XML file! It works immediately after a segment refresh (i.e., clicking up to the previous segment and back down).

I think the reason pre-lookup find/replace works less well is that by inserting target language bits into the src, you can then end up with strange translations, whereas with post-lookup find/replace, it literally just does a F&R on the final translation. 

My latest discovery is that it is often best to add shorter terms to your post-lookup F/R XML file as regular expressions, since otherwise the F&R operation will insert these shorter terms into longer ones, resulting in gobbledygook. 

For example, if I use:

<EditItem Enabled="true" EditItemType="plain_text">
<FindText>nis</FindText>
<ReplaceText>uitsparing</ReplaceText>
</EditItem>

I get this:

"De tweede holte heeft bij voorkeur een cilindrische of couitsparingche vorm."

~~~~~

If I use:

<EditItem Enabled="true" EditItemType="regular_expression">
<FindText>\bnis\b</FindText>
<ReplaceText>uitsparing</ReplaceText>
</EditItem>
</Items>

I get this:

"De tweede holte heeft bij voorkeur een cilindrische of conische vorm."