WiX v4: Custom bootstrapper application in a wixlib

[drolevar]

Hi,

We used a custom bootstrapper application in a wixlib in WiX v3.

This is how it was defined in a wixlib:

<?xml version="1.0" encoding="UTF-8"?>
<Wix xmlns="http://schemas.microsoft.com/wix/2006/wi">
  <Fragment>
    <BootstrapperApplication Id="MyInstaller.BootstrapperApplication" SourceFile="res\WixStdBA.dll">
      <PayloadGroupRef Id="MyInstallerPayloads"/>
    </BootstrapperApplication>
  </Fragment>
  <Fragment>
    <PayloadGroup Id='MyInstallerPayloads'>
      <Payload Name='thm.xml' Compressed='yes' SourceFile='!(wix.WixStdbaThemeXml=res\HyperlinkTheme.xml)' />
      <Payload Name='thm.wxl' Compressed='yes' SourceFile='!(wix.WixStdbaThemeWxl=res\HyperlinkTheme.wxl)' />
      <Payload Name='logo.png' Compressed='yes' SourceFile='!(wix.WixStdbaLogo=res\colours_banner.jpg)' />
    </PayloadGroup>

    <CustomTable Id='WixStdbaInformation' BootstrapperApplicationData='yes'>
      <Row>
        <Data Column='LicenseUrl'>!(wix.WixStdbaLicenseUrl)</Data>
      </Row>
    </CustomTable>
  </Fragment>
</Wix>

and used in the project like this:

    <BootstrapperApplicationRef Id="MyInstaller.BootstrapperApplication">
      <bal:WixStandardBootstrapperApplication
        LicenseUrl="https://mycompany.com"
        ShowVersion="yes"
      />
    </BootstrapperApplicationRef>

I've upgraded the definition in the wixlib to the following:

<Fragment>
    <BootstrapperApplication Id="MyInstaller.BootstrapperApplication">
      <PayloadGroupRef Id="MyInstallerPayloads" />
    </BootstrapperApplication>
    <BootstrapperApplicationRef Id="WixStandardBootstrapperApplication_X86" />
  </Fragment>
  <Fragment>
    <PayloadGroup Id="MyInstallerPayloads">
      <Payload Name="thm.xml" Compressed="yes" SourceFile="!(wix.WixStdbaThemeXml=res\HyperlinkTheme.xml)" />
      <Payload Name="thm.wxl" Compressed="yes" SourceFile="!(wix.WixStdbaThemeWxl=res\HyperlinkTheme.wxl)" />
      <Payload Name="logo.png" Compressed="yes" SourceFile="!(wix.WixStdbaLogo=res\colours_banner.jpg)" />
    </PayloadGroup>

    <BundleCustomDataRef Id="WixStdbaInformation">
      <BundleElement>
        <BundleAttribute Id="LicenseUrl" Value="!(wix.WixStdbaLicenseUrl)" />
      </BundleElement>
    </BundleCustomDataRef>
  </Fragment>

However building the corresponding bundle project fails with

Bundle.wxs(12): error WIX0005: The BootstrapperApplicationRef element contains an unexpected child element 'WixStandardBootstrapperApplication'.

Question: is it even currently supported in WiX v4? Or am I doing it fundamentally wrong and there's another way of implementing it in WiX v4? My goal is to have a custom bootstrapper application in a wixlib which is based on WixStandardBootstrapperApplication and differs only by theming. Previously I was using a custom wixstdba.dll, now I don't need to do that.
The idea is to reference the custom application with minimum configuration required in the bundle project.