The scenario you've described can be solved using this work around, if your dashboard is not too big. If the time for the trendInterval is from a finite set, then you can have different panels for each trendInterval, and toggle their appearance by the use of tokens that can be set and unset. For example, if the time intervals you are interested in are 24hr and 1week, you can do the following:
<input type="dropdown" token="time_span" searchWhenChanged="true">
...
<choice value="24hr">24 Hour</choice>
<choice value="1w">7 Days</choice>
...
<change>
<condition value="24hr">
<unset token="1w_token"></unset>
<set token="24hr_token"></set>
</condition>
<condition value="1w">
<unset token="24hr_token"></unset>
<set token="1w_token"></set>
</condition>
</change>
</input>
With that in place, you can use:
<panel depends="$24hr_token$, $other_tokens...$">
...
<single>
<option name="trendInterval">-24hr</option>
</single>
...
</panel>
And similarly for the 7 day time interval.
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