I have figured out a solution for this! It's not perfect, but it's okay. See the word mānuka as an example: https://www.sketchstudio.co.nz/portfolio/amber-valley
First, find a font that looks similar to your font, but supports macrons. Add the font to your website's custom fonts. Bold all letters with macrons and use the following code to replace them with the other font:
#ADDBLOCKID strong {
font-weight: 200 !important;
font-family: 'FONT' !important;
font-size: 0.8rem !important;
letter-spacing: 0.05em !important;
}
I hope this helps someone out there!