Assuming your site collections are in same web application, you can easily use xml sitemap.
Below are the steps:
1) Create a xml sitemap(myxml.sitemap for example) at root web application directory.
2)In web.config, under providers section, add :
<add name="MySPXmlContentMapProvider" siteMapFile="myxml.sitemap" type="Microsoft.SharePoint.Navigation.SPXmlContentMapProvider, Microsoft.SharePoint, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" />
3) In master page, you can bind the provider to SiteMapDataSource control and use this datasource for menu:
<asp:SiteMapDataSource ShowStartingNode="False" SiteMapProvider="MySPXmlContentMapProvider" id="xmlSiteMap" runat="server"/>
<Sharepoint:AspMenu ID="TopNavigationMenuV4" Runat="server" DataSourceID="xmlSiteMap" UseSimpleRendering="true" UseSeparateCss="false" Orientation="Horizontal" StaticDisplayLevels="1" MaximumDynamicDisplayLevels="3" SkipLinkText="" CssClass="s4-tn" />
If your site collections are in different web applications, you can store the sitemap information to a central DB or XML file and create a WCF service to read it. The WCF service can be used as datasource via SharePoint designer.