With the assistance of respondants here and in other forums, I found a simple hack that seems to solve my problem. It came to me after I gave up the fight with SSRS. I'm sure that there are lots of perfectly valid reasons as to why SSRS handles different resolution parameters as it does. Frankly, I don't give a hoot. I just need my document generated at the size and resolution I specified.
Here's the relevant pieces of code (error handling removed for the sake of brevity). I invoke SSRS via the web service interface, generate the report, and render it as 300 x 300 TIFF. The results are temporarily saved. It will have been generated as a 96ppi TIFF and scaled up. I then read it in into a BitMap, change the resolution to 300 x 300, and write it back out again. Done.
string deviceInfo = "<DeviceInfo> <OutputFormat>TIFF</OutputFormat> <DpiX>300</DpiX> <DpiY>300</DpiY> <PrintDpiX>300</PrintDpiX> <PrintDpiY>300</PrintDpiY> </DeviceInfo>";
string format = "IMAGE";
Byte[] results;
string mimeType = "image/tiff";
// Generate the report as a TIF
ExecutionInfo ei = new ExecutionInfo();
ei = rsExec.LoadReport(_reportName, historyID);
results = rsExec.Render(format, deviceInfo, out extension, out encoding, out mimeType, out warnings, out streamIDs);
string TempFileRSGeneratedTIFF = TempPath + "RSGeneratedTIFF.TIF";
// Save tiff file returned by RS
using (FileStream stream = File.OpenWrite(TempFileRSGeneratedTIFF))
{
stream.Write(results, 0, results.Length);
}
// Read tif file into bitmap
Bitmap image = new Bitmap(TempFileRSGeneratedTIFF);
// Change the resolution to what it was supposed to be in the first place..
image.SetResolution(300, 300);
// Save the final version of the file
image.Save(DestFileName, System.Drawing.Imaging.ImageFormat.Tiff);
image.Dispose();