Thorsten Scherf tested this scenario with a RHEL-7.3 system with following conf:

$ grep -A4 dynamic-db /etc/named.conf
dynamic-db "ipa" {
    library "ldap.so";
    arg "uri ldapi://%2fvar%2frun%2fslapd-DOMAIN-COM.socket";
    arg "base cn=dns, dc=domain,dc=com";
    arg "fake_mname vm-96.domain.com.";

And it worked for him.

[root@vm-idm-031 ~]# cat /etc/named.conf
.
.
dyndb "ipa" "/usr/lib64/bind/ldap.so" {
	uri "ldapi://%2fvar%2frun%2fslapd-TESTRELM-TEST.socket";
	base "cn=dns, dc=testrelm,dc=test";
	server_id "master.testrelm.test";
	auth_method "sasl";
	sasl_mech "GSSAPI";
	sasl_user "DNS/master.testrelm.test";
	fake_mname "reachable.master.test";
};
[root@vm-idm-031 ~]# systemctl restart named-pkcs11
[root@vm-idm-031 ~]# dig +short -t SOA testrelm.test.
master.testrelm.test. hostmaster.testrelm.test. 1515074331 3600 900 1209600 3600
[root@vm-idm-031 ~]# dig +short -t SOA master.test.
[root@vm-idm-031 ~]# 
[root@vm-idm-031 ~]# cat /etc/redhat-release 
Red Hat Enterprise Linux Server release 7.5 Beta (Maipo)

As part of DNS Locations https://pagure.io/freeipa/issue/2008 it is possible to configure it in LDAP in per DNS server configuration.

design page:
* https://docs.pagure.org/bind-dyndb-ldap/Design/PerServerConfigInLDAP.html
* https://www.freeipa.org/page/V4/DNS_Location_Mechanism#DNS_server_configuration
doc: https://pagure.io/bind-dyndb-ldap

Can be set in CLI by:

$ ipa dnsserver-mod ipa.myipa.test --soa-mname-override fake.myipa.test.

dig +short  -t SOA myipa.test
fake.myipa.test. hostmaster.myipa.test. 1515083621 3600 900 1209600 3600

Setting fake_mname in /etc/named.conf didn't work for me as well but that is expected as /etc/named.conf has lower priority than the LDAP entry according to design page:

"""
Priority order for configuration

Highest priority is on top:

    DNS zone: idnsZone entry
        SOA MNAME is an exception - server’s config overrides zone’s config
    per-server config in LDAP: idnsServerConfig entry
    global config in LDAP: idnsConfig entry
    per-server config on disk: named.conf
    built-in defaults
"""

Migration should be handled as well:

"""
Upgrade

Following options from named.conf should be migrated to newly created idnsServerConfigObject during IPA upgrade:

    fake_mname -> idnsSOAMName
    forward -> idnsForwardPolicy
    forwarders -> idnsForwarders

At the same time, named.conf needs to be updated to contain server_id option equal to FreeIPA server’s FQDN without trailing period (for consistency with names used in cn=masters).
"""

I did not test the upgrade though.

The only thing to improve I see is bind-dyndb-ldap documentation which doesn't mention the priorities (it is only on the design page).

I committed a documentation update upstream with https://pagure.io/bind-dyndb-ldap/pull-request/178 to clarify the order priorities after checking it out on a test deployment and also with one of upstream users.

Clarification from freeipa-users@ can be seen at https://lists.fedoraproject.org/archives/list/freeipa-users@lists.fedorahosted.org/message/UKMCM5I42M4NS3BLR7T5VH2XHGTA4QE6/

Thank you taking your time and submitting this request for Red Hat Enterprise Linux 7. Unfortunately, this bug cannot be kept even as a stretch goal and was postponed to RHEL8.

Verified.  Following is the snippets picked from runner.log

IPA Version:

2020-08-06T07:29:40+0000 ok: [master.testrealm.test] => (item=ipa-server) => 
2020-08-06T07:29:40+0000   msg:
2020-08-06T07:29:40+0000   - arch: x86_64
2020-08-06T07:29:40+0000     epoch: null
2020-08-06T07:29:40+0000     name: ipa-server
2020-08-06T07:29:40+0000     release: 8.module+el8.3.0+7513+a375844a
2020-08-06T07:29:40+0000     source: rpm
2020-08-06T07:29:40+0000     version: 4.8.7

pytest summary 

2020-08-06T07:39:52+0000 ------------------ generated xml file: /tmp/wp/twd/junit.xml -------------------
2020-08-06T07:39:52+0000 ------------- generated html file: file:///tmp/wp/twd/report.html --------------
2020-08-06T07:39:52+0000 ==================== 1 passed, 1 warnings in 515.40 seconds ====================

Test added upstream:
master:
https://pagure.io/freeipa/c/592f3fe65986077ec0a19158d061a371c86307c7

Test added upstream:
ipa-4-8:
https://pagure.io/freeipa/c/b93f2a70d09f44a64a320ad558c76536e9b625ad

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (Moderate: idm:DL1 and idm:client security, bug fix, and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2020:4670