The XllRtdLink module is a very simple implementation of Excel's RTD interface, built using ATL. It's code is based on the Microsoft Visual Basic 6 sample, "How To Create a RealTimeData Server for Excel", which can be found at http://support.microsoft.com/kb/285339/EN-US/.
In the retail vesrion of XLL+, the source code is provided in the directory Samples\XllRtdLink.