You can add it through XML and just make a config setting that enables/disables it.
<default>
<reference name="top.links">
<action method="addLink" translate="label title" module="[module]" ifconfig="[module]/offer/enabled">
<label>Offer of the day</label>
<url>URL HERE</url>
<title>Offer of the day</title>
<prepare/>
<urlParams/>
<position>100</position>
</action>
</reference>
</default>
Then create in your module inside system.xml
this setting:
<config>
<sections>
<[module] translate="label" module="[module]">
<label>[Module]</label>
<tab>general</tab>
<frontend_type>text</frontend_type>
<sort_order>1000</sort_order>
<show_in_default>1</show_in_default>
<show_in_website>1</show_in_website>
<show_in_store>1</show_in_store>
<groups>
<offer translate="label">
<label>Offer of the day</label>
<frontend_type>text</frontend_type>
<sort_order>10</sort_order>
<show_in_default>1</show_in_default>
<show_in_website>1</show_in_website>
<show_in_store>1</show_in_store>
<fields>
<enabled translate="label">
<label>Enable</label>
<frontend_type>select</frontend_type>
<source_model>adminhtml/system_config_source_yesno</source_model>
<sort_order>10</sort_order>
<show_in_default>1</show_in_default>
<show_in_website>1</show_in_website>
<show_in_store>1</show_in_store>
</enabled>
</fields>
</offer>
</groups>
</[module]>
</sections>
</config>
Replace [module]
with your module name