Based on the documentation linked by @JasSuri, I was able to come up with a solution.
To implement this, you modify the sign-up technical profile. You must add three things:
- Add item 
IncludeClaimResolvingInClaimsHandling with value true to the metadata 
- Add a 
DefaultValue attribute to the email input claim with an appropriate claims resolver notation as the value 
- Add an 
AlwaysUseDefaultValue attribute to the email input claim with true as the value 
I used an Oauth2 key-value claims resolver (which supports arbitrary query string parameters) and a query parameter named register_email.
The resulting technical profile looks like this:
<TechnicalProfile Id="LocalAccountSignUpWithLogonEmail">
  <DisplayName>Email signup</DisplayName>
  <Protocol Name="Proprietary" Handler="Web.TPEngine.Providers.SelfAssertedAttributeProvider, Web.TPEngine, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
  <Metadata>
    <Item Key="IpAddressClaimReferenceId">IpAddress</Item>
    <Item Key="ContentDefinitionReferenceId">api.localaccountsignup</Item>
    <Item Key="language.button_continue">Create</Item>
    <Item Key="IncludeClaimResolvingInClaimsHandling">true</Item> <!-- ADD THIS -->
  </Metadata>
  <CryptographicKeys>
    <Key Id="issuer_secret" StorageReferenceId="B2C_1A_TokenSigningKeyContainer" />
  </CryptographicKeys>
  <InputClaims>
    <!-- ADD DefaultValue AND AlwaysUseDefaultValue ATTRIBUTES BELOW -->
    <InputClaim ClaimTypeReferenceId="email"
        DefaultValue="{OAUTH-KV:register_email}"
        AlwaysUseDefaultValue="true" />
  </InputClaims>
  <OutputClaims>
    <OutputClaim ClaimTypeReferenceId="objectId" />
    <OutputClaim ClaimTypeReferenceId="email" PartnerClaimType="Verified.Email" Required="true" />
    <OutputClaim ClaimTypeReferenceId="newPassword" Required="true" />
    <OutputClaim ClaimTypeReferenceId="reenterPassword" Required="true" />
    <OutputClaim ClaimTypeReferenceId="executed-SelfAsserted-Input" DefaultValue="true" />
    <OutputClaim ClaimTypeReferenceId="authenticationSource" />
    <OutputClaim ClaimTypeReferenceId="newUser" />
  </OutputClaims>
  <ValidationTechnicalProfiles>
    <ValidationTechnicalProfile ReferenceId="AAD-UserWriteUsingLogonEmail" />
  </ValidationTechnicalProfiles>
  <UseTechnicalProfileForSessionManagement ReferenceId="SM-AAD" />
</TechnicalProfile>