Update: For JReviews 2.4, you can do this by simply adding this line of code to your listings template:
<?php $Widgets->favorite($listing);?>
--Original Post--
JRreviews version: 2.3.x
This should work in any custom listings page. I figured it out by looking at the code in the com_jreviews/jreviews/views/helpers/widgets.php file.
I'm going to use the template file listings_blogview.thtml as an example. Start by copying the file from the default template to your overrides folder, and giving it an _extension if you only want to the hack to apply to certain pages (or you can give it no extension and it will override default theme file). That means:
Copy from com_jreviews/jreviews/views/themes/default/listings/listings_blogview.thml to templates/jreviews_overrides/views/themes/my_theme/listings/listings_blogview_myextension.thtml
In JReviews backend, click the Clear File Registry button so that JReviews is aware of the new file in the jreviews_overrides folder. See the Documentation for setting up you theme overrides folder and configuration.
In the new custom theme file, find the favorites code
<?php if($this->Config->favorites_enable):?>
<span class="jrFavoriteWidget" title="<?php __t("Favorites") ?>">
<span class="jrIcon jrIconFavorites"></span>
<span id="jr_favoriteCount<?php echo $listing['Listing']['listing_id'];?>"><?php echo $listing['Favorite']['favored'];?></span>
</span>
<?php endif;?>
And replace it with:
<?php if($this->Config->favorites_enable):?>
<span class="jrFavoriteWidget" title="<?php __t("Favorites") ?>">
<span class="jrIcon jrIconFavorites"></span>
<span id="jr_favoriteCount<?php echo $listing['Listing']['listing_id'];?>"><?php echo $listing['Favorite']['favored'];?></span>
</span>
<?php if($listing['Favorite']['my_favorite']): ?>
<span id="jr_favoriteImg<?php echo $listing['Listing']['listing_id'];?>" class="jrFavoriteButton jrButton" title="<?php __t("Remove from favorites");?>" onclick="jreviews.favorite.remove(this,{'listing_id':'<?php echo $listing['Listing']['listing_id'];?>'})"> <?php __t("Remove");?> </span>
<?php elseif($User->id): ?>
<span id="jr_favoriteImg<?php echo $listing['Listing']['listing_id'];?>" class="jrFavoriteButton jrButton" title="<?php __t("Add to favorites");?>" onclick="jreviews.favorite.add(this,{'listing_id':'<?php echo $listing['Listing']['listing_id'];?>'})"> <?php __t("Add");?> </span>
<?php else: ?>
<span id="jr_favoriteImg<?php echo $listing['Listing']['listing_id'];?>" class="jrFavoriteButton jrButton" title="<?php __t("Add to favorites");?>" onclick="alert('<?php __t("Register to add this entry to your favorites");?>');"> <?php __t("Add");?> </span>
<?php endif;?>
<?php endif;?>
Hope this helps

[Note: I updated the code to include an Add button for guests, which pops up with a registration alert when clicked]