gwt:*ErrorFn doc, and override browser user-agent detection?

In Gmail, for instance (yes, I know it predates GWT, but please bear
with me for a moment), it's possible to override browser detection and
run against an unsupported browser for various reasons. So I've been
dissecting the nocache.js in Detailed compile mode to see if this sort
of thing might be possible in GWT.

First thing I noticed was that the meta tags "gwt:onLoadErrorFn" and
"gwt:onPropertyErrorFn" are not officially documented at all, and only
mentioned in passing in a few threads here. They seem pretty
straightforward, and here's what they appear to do upon human-eye
interpretation. (Please feel free to copy this to the official doc and
expand upon them, as they appear to be VERY useful and deserve
official mention.)

===
gwt:onLoadErrorFn:
Javascript function name in page global scope, which is called in
the host page's onLoad hook when loading of the cacheable bits fails.
Called with two arguments: the rename-to name of the module; and the
base URI of the host page (calculated from the topmost included script
file URI; if that can't be determined, uses the <BASE HREF> value of
the host page if present; and finally uses the host page URI with any
filename part trimmed as a last resort).

gwt:onPropertyErrorFn:
Javascript function name in page global scope, which is called when
a GWT global property value falls outside the set of allowed values
for that property.
Called with three arguments: the property name; an array of the
allowed values for that property; and the invalid value.
===

So if an unknown browser type is detected, what happens is that the
gwt:onPropertyErrorFn is called with the arguments ('user.agent',
[...known permutations...], 'unknown'). OK, so implementing a hook to
do something useful on an unknown browser is pretty easy.

Here's the harder part that I can't figure out: Is there a way to
specify that certain User-Agents (that aren't already hardcoded into
GWT's detection code) map to a given user.agent value? I see the
<property-provider> code in com/google/gwt/user/UserAgent.gwt.xml, but
that seems pretty much fixed in stone.

The idea would be to have additional code that I could use to map
user.agent values, coercing currently-unsupported browsers into a
given JS dialect to provide at least some application usability. Is
there any way to extend that code in UserAgent.gwt.xml without copying
it verbatim or hacking it up in-place?

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