If you replaced the contents of $:/core/ui/Buttons/new-journal
with
\define journalButton()
<$button tooltip={{$:/language/Buttons/NewJournal/Hint}} aria-label={{$:/language/Buttons/NewJournal/Caption}} class=<<tv-config-toolbar-class>>>
<$action-sendmessage $message="tm-new-tiddler" title=<<now "$(journalTitleTemplate)$">> tags="$(journalTags)$" text="$(journalText)$"/>
<$list filter="[<tv-config-toolbar-icons>prefix[yes]]">
{{$:/core/images/new-journal-button}}
</$list>
<$list filter="[<tv-config-toolbar-text>prefix[yes]]">
<span class="tc-btn-text"><$text text={{$:/language/Buttons/NewJournal/Caption}}/></span>
</$list>
</$button>
\end
<$set name="journalTitleTemplate" value={{$:/config/NewJournal/Title}}>
<$set name="journalTags" value={{$:/config/NewJournal/Tags}}>
<$set name="journalText" value={{$:/config/NewJournal/Text}}>
<<journalButton>>
</$set></$set>
then you could put your default text in
$:/config/NewJournal/Text
and have it available as you needed it. Of course, you might miss any upgrades to New Journal and you'll need to remember the name of the tiddler where you stowed the default text. I guess the next step would be to add a box to the configuration screen. I'm kind of surprised that that isn't already available.
Oh yeah - be sure to make a backup before trying this!
HTH
Mark