I'm using the Open Office Calc engine under the hood for Excel formula emulation. I extend that with my own implementation of the Excel4 API's xlcall32.dll, so that XLLs can invoke the Excel4 API. I also emulate Excel's built in RTD client, so that XLLs that supply an RTD server can push real time data into a sheet. Feel free to download the install kit [1], or email me on ssbeta@spreadserve.com for more detailed discussion. Thanks for the interest!

[1] http://spreadserve.com/s3/downloads.html