How to include a modifiable loose file in WiX burn?

I have a setup package including a non-compressed file.

<DirectoryRef Id="INSTALLLOCATION">
  <Component Id="LocationConfig">
    <File Id="LocationConfigFile" 
          Source="LooseFile.Config" 
          DiskId="2" 
          Vital="no" 
          Compressed="no" />
  </Component>
</DirectoryRef>

The purpose of this file being loose is so it can be edited before installation. This works as desired.

I have a burn chain specifying that the loose file is included as a loose file alongside the bootstrapper. Note also the use of SuppressLooseFilePayloadGeneration to allow the manual specification of the file as a loose payload.

<Chain>
  <MsiPackage SourceFile="MyInstaller.msi" 
              Visible="yes" 
              Vital="no"
              SuppressLooseFilePayloadGeneration="yes">
    <Payload Compressed="no" SourceFile="LooseFile.Config" />
  </MsiPackage>
</Chain>

The burn log looks like this:

[3860:38D8][2013-04-26T16:42:48]e000: Error 0x80091007: Hash mismatch for path: C:\ProgramData\Package Cache\.unverified\payAC32431CF002C09E2F0B537A32ACA259
[3860:38D8][2013-04-26T16:42:48]e000: Error 0x80091007: Failed to verify hash of payload: payAC32431CF002C09E2F0B537A32ACA259
[3860:38D8][2013-04-26T16:42:48]e310: Failed to verify payload: payAC32431CF002C09E2F0B537A32ACA259 at path: C:\ProgramData\Package Cache\.unverified\payAC32431CF002C09E2F0B537A32ACA259, error: 0x80091007. Deleting file.
[3860:38D8][2013-04-26T16:42:48]e000: Error 0x80091007: Failed to cache payload: payAC32431CF002C09E2F0B537A32ACA259
[33FC:3A54][2013-04-26T16:42:48]e314: Failed to cache payload: payAC32431CF002C09E2F0B537A32ACA259 from working path: C:\Users\Snixtor\AppData\Local\Temp\{c887e0cf-5038-4e15-95b1-8510d8c96b88}\payAC32431CF002C09E2F0B537A32ACA259, error: 0x80091007.

OK, the hash is failing because the file has changed. But... I want to allow the user to change the file. I can readily enough do this with a standard setup package, so what hoops do I have to jump through to get it to behave with a bootstrapper?

I found this discussion in the WiX users mailing list. Robs response of "It should just work" sounds promising, but then the discussion seems to move on to suggest it could be a bug? If the author ever raised a bug report, I can't find it.

An alternative I considered was to exclude the file altogether from the bootstrap payload, and then manually copy it over to the MSI cache path during install so the MSI can find it, though burn will never try to validate it. But the two troubles I see there are:

1. I can't find out how to discover the cache path from within my bootstrapper.
2. Even if I knew the path, I'd need to elevate the bootstrapper to copy the file. That may not be a showstopper, but I have a sneaking suspicion it might prove difficult.

This isn't supported today. Burn validates everything before placing it in the cache for security reasons. What you could do is read the external file in a custom Bootstrapper Application and store the results as persisted Variables. This will be more work but Burn just won't trust files that don't match the security hashes/signatures inserted at build time.