Let's Encrypt fix for webOS. Tested on 3.8 only, put this into /var/lib/webosbrew/init.d and you should be probably done.

50-customca

#!/bin/sh

# Directory to store overlays in (one directory structure is created per overlay configured down below)
OVERLAY_BASE=/var/lib/webosbrew/customca

overlay() {
        set -e
        overlay_id="$(echo $1 | sed 's;/;__;g')"
        unset TARGET SOURCE FSTYPE OPTIONS
        eval $(findmnt -P $1)
        if [[ "$FSTYPE" == "overlay" ]] || [ -f "$1" ] && [[ "$FSTYPE" != "" ]]; then
                echo "[-] Overlay '$1' already mounted"
        elif [ -f "$1" ]; then
		if [ ! -f "$OVERLAY_BASE/$overlay_id" ]; then
			echo "[ ] Preparing overlay for '$1'"
			cp $1 $OVERLAY_BASE/$overlay_id;
		fi
		mount --bind "$OVERLAY_BASE/$overlay_id" "$1"
		echo "[+] File overlay '$1' mounted"
	else
                echo "[ ] Preparing overlay for '$1' -> $OVERLAY_BASE/$overlay_id"
                mkdir -p "$OVERLAY_BASE/$overlay_id/upper" "$OVERLAY_BASE/$overlay_id/work"
                mount -t overlay -o lowerdir=$1,upperdir=$OVERLAY_BASE/$overlay_id/upper/,workdir=$OVERLAY_BASE/$overlay_id/work/ overlay-$overlay_id $1
                echo "[+] Overlay '$1' mounted"
	fi
}

# Usage:
overlay /etc/ssl/certs
overlay /usr/share/ca-certificates
overlay /etc/ca-certificates.conf
overlay /etc/pki

if [ ! -f "/usr/share/ca-certificates/mozilla/ISRG_Root_X1.crt" ]; then
  # from https://letsencrypt.org/certs/isrgrootx1.pem
  cat <<EOF >"/usr/share/ca-certificates/mozilla/ISRG_Root_X1.crt"
-----BEGIN CERTIFICATE-----
MIIFazCCA1OgAwIBAgIRAIIQz7DSQONZRGPgu2OCiwAwDQYJKoZIhvcNAQELBQAw
TzELMAkGA1UEBhMCVVMxKTAnBgNVBAoTIEludGVybmV0IFNlY3VyaXR5IFJlc2Vh
cmNoIEdyb3VwMRUwEwYDVQQDEwxJU1JHIFJvb3QgWDEwHhcNMTUwNjA0MTEwNDM4
WhcNMzUwNjA0MTEwNDM4WjBPMQswCQYDVQQGEwJVUzEpMCcGA1UEChMgSW50ZXJu
ZXQgU2VjdXJpdHkgUmVzZWFyY2ggR3JvdXAxFTATBgNVBAMTDElTUkcgUm9vdCBY
MTCCAiIwDQYJKoZIhvcNAQEBBQADggIPADCCAgoCggIBAK3oJHP0FDfzm54rVygc
h77ct984kIxuPOZXoHj3dcKi/vVqbvYATyjb3miGbESTtrFj/RQSa78f0uoxmyF+
0TM8ukj13Xnfs7j/EvEhmkvBioZxaUpmZmyPfjxwv60pIgbz5MDmgK7iS4+3mX6U
A5/TR5d8mUgjU+g4rk8Kb4Mu0UlXjIB0ttov0DiNewNwIRt18jA8+o+u3dpjq+sW
T8KOEUt+zwvo/7V3LvSye0rgTBIlDHCNAymg4VMk7BPZ7hm/ELNKjD+Jo2FR3qyH
B5T0Y3HsLuJvW5iB4YlcNHlsdu87kGJ55tukmi8mxdAQ4Q7e2RCOFvu396j3x+UC
B5iPNgiV5+I3lg02dZ77DnKxHZu8A/lJBdiB3QW0KtZB6awBdpUKD9jf1b0SHzUv
KBds0pjBqAlkd25HN7rOrFleaJ1/ctaJxQZBKT5ZPt0m9STJEadao0xAH0ahmbWn
OlFuhjuefXKnEgV4We0+UXgVCwOPjdAvBbI+e0ocS3MFEvzG6uBQE3xDk3SzynTn
jh8BCNAw1FtxNrQHusEwMFxIt4I7mKZ9YIqioymCzLq9gwQbooMDQaHWBfEbwrbw
qHyGO0aoSCqI3Haadr8faqU9GY/rOPNk3sgrDQoo//fb4hVC1CLQJ13hef4Y53CI
rU7m2Ys6xt0nUW7/vGT1M0NPAgMBAAGjQjBAMA4GA1UdDwEB/wQEAwIBBjAPBgNV
HRMBAf8EBTADAQH/MB0GA1UdDgQWBBR5tFnme7bl5AFzgAiIyBpY9umbbjANBgkq
hkiG9w0BAQsFAAOCAgEAVR9YqbyyqFDQDLHYGmkgJykIrGF1XIpu+ILlaS/V9lZL
ubhzEFnTIZd+50xx+7LSYK05qAvqFyFWhfFQDlnrzuBZ6brJFe+GnY+EgPbk6ZGQ
3BebYhtF8GaV0nxvwuo77x/Py9auJ/GpsMiu/X1+mvoiBOv/2X/qkSsisRcOj/KK
NFtY2PwByVS5uCbMiogziUwthDyC3+6WVwW6LLv3xLfHTjuCvjHIInNzktHCgKQ5
ORAzI4JMPJ+GslWYHb4phowim57iaztXOoJwTdwJx4nLCgdNbOhdjsnvzqvHu7Ur
TkXWStAmzOVyyghqpZXjFaH3pO3JLF+l+/+sKAIuvtd7u+Nxe5AW0wdeRlN8NwdC
jNPElpzVmbUq4JUagEiuTDkHzsxHpFKVK7q4+63SM1N95R1NbdWhscdCb+ZAJzVc
oyi3B43njTOQ5yOf+1CceWxG1bQVs5ZufpsMljq4Ui0/1lvh+wjChP4kqKOJ2qxq
4RgqsahDYVvTH9w7jXbyLeiNdd8XM2w9U/t7y0Ff/9yi0GE44Za4rF2LN9d11TPA
mRGunUHBcnWEvgJBQl9nJEiU0Zsnvgc/ubhPgXRR4Xq37Z0j4r7g1SgEEzwxA57d
emyPxgcYxn/eR44/KJ4EBs+lVDR3veyJm+kXQ99b21/+jh5Xos1AnX5iItreGCc=
-----END CERTIFICATE-----
EOF
  ln -sf /usr/share/ca-certificates/mozilla/ISRG_Root_X1.crt /etc/ssl/certs/ISRG_Root_X1.crt
fi

if grep -i DST_Root_CA_X3 /etc/ca-certificates.conf; then
  echo "[+] Removing DST_Root_CA_X3"
  sed '/DST_Root_CA_X3/d' /etc/ca-certificates.conf > /tmp/c && cp /tmp/c /etc/ca-certificates.conf
fi

if ! grep 'ISRG_Root_X1' /etc/ca-certificates.conf; then
  echo "[+] Adding ISRG_Root_X1"
  echo 'mozilla/ISRG_Root_X1.crt' >> /etc/ca-certificates.conf
fi

if [[ "$(ls -td /etc/ssl/certs/* /etc/ca-certificates.conf | head -1)" != "/etc/ssl/certs/ca-certificates.crt" ]]; then
  update-ca-certificates
  c_rehash
  cp /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/trusted_cas.crt 
fi

# TODO: certutil -d /etc/pki/nssdb/ -A -t 'C,,' -n CA -i /usr/share/ca-certificates/...

@Informatic
Hello! I can confitm it works on WebOS 4.9. Thank you very much!

But is there a way to make writable the inital root directory "/" ?
I tried overlay / but it shows:
[-] Overlay '/' already mounted

I need to create /opt ...

Thank you!