Can I set an arbitrary nvram variable using rEFInd?
rEFInd will not let you set an arbitrary NVRAM variable directly but you can use EDK2 to compile a small EFI driver. This is described in the WikiLeaks document EFI Basics: NVRAM Variables
EFI drivers placed in rEFInd driver directory are loaded and run automatically - see The rEFInd Boot Manager: Using EFI Drivers.  Alternatively you could write a EFI program rather than driver and then use a manual stanza in rEFInd to call a shell script to call this and then the bootloader.
Another (probably easier) method would be to modify rEFInd.  You can see in the source how it sets the csr-active-config NVRAM variable to turn SIP on and off - apple.h defines the GUID:
// The constants related to Apple's System Integrity Protection (SIP)....
#define CSR_GUID { 0x7c436110, 0xab2a, 0x4bbb, { 0xa8, 0x80, 0xfe, 0x41, 0x99, 0x5c, 0x9f, 0x82 } };

apple.c calls the EfivarSetRaw() function in lib.c
Status = EfivarSetRaw(&amp;CsrGuid, L&quot;csr-active-config&quot;, (CHAR8 *) &amp;TargetCsr, 4, TRUE);
if (Status == EFI_SUCCESS)
    RecordgCsrStatus(TargetCsr, TRUE);
else
    SPrint(gCsrStatus, 255, L&quot; Error setting System Integrity Protection code.&quot;);

Other NVRAM updates could be made in the same way - there are various other GUID listed in the WikiLeaks link in the first paragraph.

If you want to pass a boot argument without updating NVRAM you can create a manual boot entry in rEFInd and specify the options in refind.conf.  Additional options can be added using add_options - see the Creating Manual Boot Stanzas page of the documentation.
For example you can define submenu options like this and pick between them using F2 at boot.
menuentry &quot;Catalina&quot; {
    # Get loader GUID from macOS subvolume: diskutil info disk1s5|grep 'Volume UUID'
    icon \EFI\refind\themes\colourful\myicons\os_mac_silver.png
    volume &quot;Preboot&quot;
    loader \3B4B18C9-C57D-4F98-9168-C8D8B9F06EAD\System\Library\CoreServices\boot.efi
    submenuentry &quot;Verbose&quot; {
        add_options &quot;-v&quot;
    }
    submenuentry &quot;Single User&quot; {
        add_options &quot;-v -s&quot;
    }
    submenuentry &quot;Safe Mode&quot; {
        add_options &quot;-v -x&quot;
    }
    submenuentry &quot;Recovery&quot; {
        volume &quot;Recovery&quot;
        loader \3B4B18C9-C57D-4F98-9168-C8D8B9F06EAD\boot.efi
    }
}